The first thing I would try would be ctrl-alt-backspace – This will restart your Xserver, which is similar to logging out.

If that doesn’t work, holding ctr-alt-delete will restart your computer.

Sometimes, however, even these two commands don’t work. If this happens to you, the following command should be your last resort before actually holding down the power button.

Hold down alt-sysrq (sysrq is sometimes the same as the print screen button) and then type r-e-i-s-u-b (I remember it as ‘busier’ backwards)

This is what that last command actually means:

ALT+SYSRQ+R (should give back control of the keyboard )

ALT+SYSRQ+E (Sends term to all processes but init)

ALT+SYSRQ+I (Sends kill to all processes but init)

ALT+SYSRQ+S (issues a sync)

ALT+SYSRQ+U (file systems are mounted read only, prevents fsck at reboot)

ALT+SYSRQ+B (reboots the system, without the previous steps this would be a hard reset).