Help me find a book where a fantasy world exists in the ashes of a sci fi civilization

I love both sci fi and fantasy as genres and recently I was thinking how cool it would be if I was to read a book where a character in a fantasy world discovers ancient technology far superior to anything they know and is evidence of an old, science fiction society. Perhaps a mystery of what happened to this old society would also be cool.

I don't have any books similar to what I'm thinking about on hand, so I'm curious if anyone has any recommendations.