+1 to ctrl-alt-fsomething (start at f1 and go up to move through the different virtual terminals). once in a while there are graphics problems which this will fix.

If you're using GNOME Shell on X you can reload the shell (and all of its extensions) with alt-f2 and then in the "Run a command" dialog that appears type r and hit enter. Unfortunately this doesn't work in GNOME on Wayland.