The criminally underwatched "See" is one such fiction. It might be somewhat cheesy but I loved the worldbuilding.

Check out /A Canticle for Leibowitz/. Each time CollapseOS comes up, I picture a team of monks cycling alternators around the clock to keep a system powered while another compares parchment to a vacuum-tube display.

I think for its chosen niche, the key qualities are (1) low power draw and (2) the ability to boot and be usable with no on-disk storage. The z80 system has that, but you could build something much more powerful. Burn the OS into a rom, have it copy into ram at boot.

It's not quite a medieval society, but The Windup Girl https://www.goodreads.com/cs/book/show/6597651-the-windup-gi... feature a post-climate change world where most energy comes from beasts of burden, but there are still a few rare computers and staggeringly expensive carbon-fuelled vehicles.

I found Souls in the Great Machine by Sean McMullen to be an interesting take on this.

https://en.wikipedia.org/wiki/A_Connecticut_Yankee_in_King_A... isn't quite that but is a fun take on the time traveller, by Mark Twain.

You might want to check out Eric Flint’s 1632 series. A small Virginia mining town is sent back in time and translocated to 1631 Germany, the series goes into how they try to build up a more technologically advanced civilization. S. M. Stirling‘s Nantucket series is somewhat similar. Nantucket is sent back in time to 1250 BC and they try to build a society there. David Weber’s Safehold series has a different premise, it’s set in the far future on a planet with medieval level technology. A lot of it is about the protagonists trying to build up their tech level.