Re-start Linux X-Server without Rebooting
There are alternatives; the shortcut keys Alt+PrintScreen+K also quits X-windows to go out to the log-in screen without a reboot.
The Ctrl+Alt+Backspace key combination does the same, but that is often disabled in some Linux distributions. It is easily enabled:
Go to System > Preferences > Keyboard.
Select the 'Layouts' tab then the 'Options' button.
Select “Key sequence to kill the X server” and enable “Control + Alt + Backspace”.
All this does is get you out to a point where you can re-start the X-server, it's up to you to diagnose how the lock-up occurred in the first place. RC