I'm sorry to hear this. Some times cats that get along well will just out of the blue start fighting. This has happened to me many times. As another PT member has already said, maybe there's someting physically wrong with your female cat that your male can smell but you can't. I would take her to the vet to get checked out just in case. You can also try using feliway/comfort zone plugins or spray which you can buy online or at most pet stores. Hopefully they'll get along well again soon. Good luck.