Resetting your machine.

If your mouse or keyboard doesn't respond, try Ctrl+Alt+BackSpace to restart your X. Or, if you can login other machine, try to login your machine remotely and restart X or gdm by kill command kill -HUP <PID>.

For emergency, such as after hours or weekend, here is a safeway to reboot your machine if you can.

(1).

Push Ctrl+Alt+F1 to change to virtual terminal mode.

(2).

Push Ctrl+Alt+Del to reboot the system.