Are you using Windows 10? Then most likely it was some forced Windows update which included new drivers for your AMD GPUs. I ran into this problem a few times in the past. It would either not hash at all because all the overclocking settings were reset OR it would hash at a slow speed because compute mode was disabled by default.

It was a pain in the butt to disable but somehow with some reg hack I was able to prevent any updates from happening again. Search the web and there is a guide on how to disable Windows 10 updates. Its pretty indepth and you need to block access to certain DNS hosts.