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:

https://superuser.com/questions/1149165/altgr-randomly-stops-working-on-windows-10