This is a short story that I read when I was a teenager in the 80s. It is about this man who enters a regular multi-storied drugstore. he goes up to the top floor where the book section is. He buys a book and starts reading it while going down the escalator. After a few minutes of this, he realises that he is standing on an infinite escalator. The story goes on about his attempts to escape this place.
This is Thomas M Disch's Descending
A chap goes to a store for his shopping, and gets distracted on the way out, reading Vanity Fair. When he gets to the bottom of each escalator, he's turning onto the next one automatically, not realising that he's descended rather too far.
Most of the story deals with him figuring out how he's going to walk up an unknown quantity of down escalators to get back out again.