I'm looking for a Sci Fi short story about a sci fi writer in the 1800s who tells a story about going back in time using a mirror. He describes to his friends the barbaric, primitive people he met there. It goes on for a while with his descriptions, then a friend asks him how he ever got back. He answers, "I never did get back".
The only story I can think of that has an author go back in time using a mirror is L. Sprague de Camp's "Balsamo's Mirror" (1976), where H.P. Lovecraft and a friend are sent back to the 18th century (which is close to what you remember) by a gypsy using a magic mirror.
They don't get trapped there, though; they come back when their 18th-century bodies are killed, and the mirror is broken.