S.M. Stirling. What if. In this case, what if electricity and explosives suddenly ceased to work. That part’s never explained (there’s a blinding white flash that everyone sees)–space aliens? military experiment gone bad? magic? the gods? (One of the major groups in the novel are wiccans.) Lots of gore and fighting, a fair amount of idealism, and of course Bad Guys drunk on power.
Kenneth Sutton's aide-mémoire