Several restarts later, the Windows key has started working again. I guess this one remains a mystery.   
The Windows key has stopped working on my m15 R6 laptop. It was working last night when I took a screenshot with Win + Shift +S, but today it's not. I know that, for most people, when this happens it's because they've accidentally hit the Fn + F6 toggle that turns the Windows key off for gamers to avoid accidental activation of it. I have tried Fn + F6 both ways but the Win key remains inactive. Also, other Fn combos work to activate full fan mode, brightness etc so I know the Fn + Function Keys combos are working. I have tried going back to my most recent Restore Point and even to older ones, but the problem remains so I assume this is not something that System Restore can correct. If I use a Logitech wireless keyboard, the Win key works when pressed on there, so it's not an issue where the Win Key itself is disabled in the OS. I tried re-installing the Intel HID Event Filter Driver.  Any ideas?
m15 R6, Windows Key stopped working, and it's not Fn + F6