Occasionally a program that's running in full screen may die on you. It might even die so hard, that it'll stay open and block your entire screen.
They'll also became immune to exiting out and alt f4.
Even if you opened task manager, you wouldn't be able to access it as it would be covered by the crashed program.
The seemingly only option would be to restart your computer.
There's actually a very easy solution that just involves opening/creating a second desktop. Just press windows + tab and you'll be able to open/create your second desktop. From there just open task manager like usual and end the bugged program.