You should check out Sterlings “DIes the Fire” and “Emberverse” series. More fantasy than sci-fi. I enjoyed them, especially the Nantucket series, because of my love of History. It’s about an event that throws the island 3500 years in the past, and how the people deal with Incas, Proto-celts, Babylonians. Hitites, Egyptians…all of the civilizations that existed at the time. Then in the Emberverse, it’s flipped. At the same moment modern Nantucket disappears, a 3500 year old Nantucket appears in the present, and all modern technology quits working. The world become Medievil in an instant.