For an expalnation see:
kember.net/.../reisub-the-gentle-linux-restart

The typical keypress is Right-Alt + SysReq + K. It should kill X and bring
you back to login. This replaced Ctrl + Alt + Backspace. Ctrl + alt + delete
typically does not help in Kubuntu. If you just want to kill a frozen
application use: Alt + F2 then type xkill & press enter. You will get a
skull and cross bones cursor icon. Click it on the frozen window (but not
your desktop!).