I remember this novel's plot in some detail, but nothing about the author, the title, or the character names. I believe this book was already on the shelves of a public library in Indiana in the mid-1980s, and I reread it roughly a decade later (when I found it in another library). I'm sure the book was a self-contained story with no signs of being part of any larger series. I'm sure it was written in English, and almost sure it was not written by any well-known, prolific SF author of the 20th Century. (Definitely not any of the ones whose works I tend to collect and reread for fun, such as Heinlein, Asimov, Dickson, Zelazny, etc.)
It starts in our future -- perhaps just a century or two after the late 20th Century. Technology has continued to advance. The main character is a restless young man who somehow gets himself into very hot water. I think this had something to do with sexually promiscuous behavior. (Perhaps sleeping with the wife, or other female relative, of a very powerful man.) I am not sure if the young man was also facing serious criminal charges, but I have an idea he may have been -- even if he wasn't guilty of everything he was accused of. Let's call him "Protagonist."
The solution that is found by a prominent politician with a soft spot for Protagonist (with a family tie of blood or of marriage, I believe -- possibly a much older brother, or an uncle) is to arrange to put Protagonist into cryogenic sleep (or the functional equivalent?) for a while -- say, a century or so -- until the heat dies down. Heck, this may actually have been the sentence formally imposed by a court of law in a plea bargain or something -- my memory of the first few chapters is particularly thin. I don't recall if Protagonist ever stood trial, or just was rightly afraid of what would happen if he insisted on doing so, or what.
The young man is put to sleep . . . and, similar to what happened to Buck Rogers, wakes up several centuries later in the middle of nowhere. In other words, in what appears to be a rural area, nowhere near anything resembling a modern city. (I don't recall how that matches up with the neighborhood around the place where he was first frozen.)
I think the young man is naked, initially is very much in shock, is found running around in the rain in the middle of a stormy night, and is treated as a gibbering lunatic at first. (Unless I'm confused with some other story.) I think that he eventually decides that at some point his frozen body had been stored in some underground facility in England (but I'm not sure about "England") which had been completely self-powered and thus, even though the surviving population of that region had long since forgotten that this facility existed, this young man had been kept safe for centuries until the forces of nature -- lightning strike during a thunderstorm, maybe? -- somehow damaged the electrical systems and caused him, as a failsafe, to defrost and wake up. (Nobody else had the same experience -- I don't recall if other frozen people were stated or implied to still be in there, possibly dying from equipment failure, at the same time he awoke.)
The world around him seems to have reverted back to the Middle Ages. I don't think the local culture even has gunpowder, but I may be wrong about that detail. I don't recall what was stated about why things had regressed -- presumably something cataclysmic had nearly destroyed human civilization, but I don't recall if anyone ever suggested it was a nuclear war. As opposed to the extravagant use of a whole different type of Weapons of Mass Destruction, or some other apocalyptic event. The world, or at least the portion of it which the protagonist is now stuck in, is essentially ruled by a theocracy which deliberately stifles any attempt to improve the available technology. (A reasonable guess would be that this taboo began because the shell-shocked survivors of a previous generation had blamed billions of deaths on high-powered technology getting out of control, and called it "the devil's tools" or something similar. But I cannot swear that this was stated in so many words.)
A local leader -- I believe he is the richest man in this rural area, and probably also a local authority figure in the all-powerful Church, sort of like "senior clergyman combined with local baron" -- takes a personal interest in this mysterious young man. I will just call this guy "Big Shot." After the young man calms down and they have a serious talk, Big Shot becomes convinced that his guest comes from the mythical times when men could fly through the air, and carry on conversations with people thousands of miles away, and so forth. In theory, this makes the protagonist a very dangerous person who could bring back the bad old days of science and technology. Running around in public, talking about the marvels he took for granted in his childhood, could cause Protagonist to be executed as a heretic.
Fortunately, Big Shot seems to have a very easygoing attitude towards the prohibitions of the church. (Even if he is, as I think, a member of its clergy.) Big Shot wants the young man to come up with something that will enable Big Shot to become much more wealthy and influential than he is now. A clerk working for Big Shot spends a long time talking to Protagonist, asking him questions and taking notes, hoping he can remember details of how to build this, that, or the other useful thing. The problem is that the young man never took much interest in such boring things as the details of chemistry and engineering; he just enjoyed the results of centuries of technological development.
Ultimately, however, one line of questioning causes Protagonist to say, "Wait a minute . . . once upon a time I looked at some diagrams for how one of the first steam engines was built. And if we built one, it could run by just burning wood; wouldn't need electricity at all!" However, he wasn't planning to ever do anything with the knowledge at the time, and he doesn't have a photographic memory for every detail.
This impasse is solved by Mystery Girl. She popped up earlier in the plot, but I think this is when she first does something important, so I didn't mention her earlier. Mystery Girl is an attractive young woman with a nice figure who was strongly hinted to have ulterior motives for working in the area as a domestic servant, or something along those lines. She might be a spy. I believe there were also previous hints that she had some sort of psychic ability to make herself seem very harmless and uninteresting, so that male authority figures didn't feel any sexual interest in her. I also think having psychic talents is forbidden by the theocracy -- witches to be burned at the stake, maybe?
Now, when the steam engine thing looks like a red-hot lead, she insists she should be left alone in a room with Protagonist. She is. We are not told what happens next. But a few hours later the young man has actually been able to recreate the drawings he once studied. (Implication: Either Mystery Girl hypnotized him in order to refresh his memory, or she used telepathy to do much the same thing, or possibly some combination of the two. I don't think it was ever explained.)
It still takes months of painstaking work, but Protagonist and a couple of trusted craftsmen finally are able to build a very nice working prototype of that old steam engine. Unfortunately, just as Big Shot is starting to realize a nice return on what he's invested in this project, it becomes clear that word has leaked out somehow, and another clergyman -- the local equivalent of a senior member of the Spanish Inquisition -- has come to investigate the rumors of strange goings-on. Things get messy. I believe the steam engine explodes, the craftsmen are killed, possibly Big Shot as well, and Protagonist and Mystery Girl (who has become his lover at some point) only manage to escape by the skin of their teeth.
Mystery Girl is now revealed to be a member of some sort of secret society which opposes the current theocratic power structure. (I don't recall if there was much evidence that their opposition had ever accomplished much.) In a surprise plot twist which I found highly contrived and unconvincing even when I first read it (around age 12 or 13, maybe?), leaders of Mystery Girl's faction suddenly reveal to Protagonist that they think they know a fancy technique which can utilize their combined psychic powers to send him back in time to his native era. And they are willing to prove it, right now! If he really wants to go, of course.
I believe they have Protagonist stand in the middle of a circle outdoors, and then the psychics start doing whatever it is that they do . . . and maybe there are visual signs that something is happening (mist springing up out of nowhere, for instance) . . . and then he suddenly realizes he'd rather stay here with the girl whom he loves, and he jumps out of the circle and the ritual accomplishes nothing. (Then Mystery Girl reveals that she is already pregnant with his child -- I don't think Protagonist or anyone else knew that before.)
I believe the story ends right after this, so we never find out if the secret society had any luck in shaking the grip of the repressive Church authorities. (Please note that I don't say it was a "Christian" church -- I can't recall if the name Jesus Christ still meant anything at all in that distant future.)
Does anyone think this sounds familiar? I recognize that the basic concept of "a fellow from our era, or our near future, suddenly finds himself in a strange and scary distant future where things have regressed" is not exactly the most original plot premise in the history of science fiction. But for some reason I feel mildly interested in pinning down who wrote this particular version of that trope. I didn't fall madly in love with the book the first or second times I read it, but it might be interesting to learn what else this particular author turned out (if anything), and give it a fair chance to impress me if I haven't read it before.