Lone fly fisherman, above timberline, caught in a storm, seeks shelter in a cave, cave is occupied by an alien astronaut who crashed on Earth long ago, alien astronaut (humanoid but in a space suit) is hibernating but is awakened by the presence of the man, alien keeps man prisoner. Can't remember the ending which is why I am trying to find it. Not a thick book - published by something like Ballantine or Ace. Cheap paperback. Cover had a picture of mountains, the fisherman and the alien astronaut in a suit and helmet. Read this over 40 years ago and it was old then.
The Winds of Time, a 1957 novel by Chad Oliver which was also my (unaccepted) answer to the old question Aliens crash on Earth and go into stasis to wait for technology to fix their ship. However, none of the covers I can find seems to quite match your description.
Wes Chase, a doctor on a Rocky Mountain fishing escape from his L.A. practice and an unloving wife, gets kidnapped by a pasty-faced character with a potent hand weapon. He is held captive in a cave containing four bodies in niches while his captor painstakingly absorbs English. From that point on, the story rightly belongs to the Intruders. They're far more interesting than Doc Chase.
They are looking for a world with a spaceship technology and we don't have it. However, there's much more to it than that.
The book starts as an adventure. Wes Chase, vacationing M.D., is caught in a storm high in the Rockies and holes up in a convenient cave to wait it out. But there's a door in the back of the cave, and through the door comes an alien from the stars, who has been sleeping there for fifteen thousand years . . .
Then, on p. 44, the focus shifts to this starman—Arvon of Lortas—and the crew of the ship in which they have been searching the Universe for other men like themselves. The Lortans, alone among the human kind who teem among the worlds, have reached an ultimate technological world-civilization without first destroying themselves in atomic or bacterial war. But they have reached a dead end; to rise higher they need the cross-fertilization of ideas shared with another human race as advanced and stable as their own. And they can't find one.
The Lortan ship is wrecked on Earth, somewhere in Siberia, late in the Wisconsin glacial period when the first roving hunters are crossing to America. There's a nice and regrettably brief bit devoted to the nameless, pragmatically friendly folk among whom they fall, but five of the ship's company decide to put themselves into a fifteen thousand-year sleep in the hope that when they emerge, Man will be ready to build them a new star-ship. Instead, they awake in our time, with the atomic issue still unsettled and human technology still too crude for a space-drive.