Mines stopped working without notice last Tuesday. I tried out most of the fixes the other replays suggest, unfortunately they didn't work; However I did notice 2 things. When I was in Device Manager uninstalling the surface type cover driver, I reconnected the type cover and the manager automatically gave me the driver back.
When I was in the UEFI, the keyboard didn't work but the touchpad worked (only in UEFI though). I could click on tabs like normal. So I figured that the Surface recognizes it connected but it doesn't recognize it properly.
Has anyone seen this too?