Comment 78 for bug 1181666

@Daniel: I still see this bug occasionally in 18.10 and 19.04. When it happens, the ubuntu dock and the status bar still work, but none of the applications will accept any input, including gnome-terminal. You can switch applications using the dock, but alt-tab doesn't do anything, and ALT-F2 doesn't work.

At this point all I have figured out to try is to go to another tty console and try "DISPLAY=:0 gnome-shell --replace&disown". When I switch back to the gnome-shell tty (usually tty2), it restarts and all is fine for a some time (just now it was fine for around 20 seconds) and then gnome-shell completely and I'm back at the login screen on tty1.