I recently had the unpleasant experience that the AltGr key stopped working. After some googling and finding many pages that talked about it being caused by Remote Desktop (which I did not use), I finally found out that simultaneously pressing:
Shift – Caps Lock – AltGr
resolved the issue. I have no idea what the purpose of this shortcut is and it seems pretty stupid, but it is good to know about it if it happens again.
The page where I found the solution is: