Despite the fact that i ran my first Linux in the middle of the 90th it took me more then 10 years to get to know how to shutdown a frozen Linux system gently. Use this when CTRL-ALT-BACKSPACE for X server restart or CTRL+ALT+F1 to get a console won't work anymore.
Simply press down the "Alt" and the "SysRq" (Print Screen) keys together. Hold'em and type the six letters
Your Linux operating system will shutdown in six steps - one for each keystroke. Here is what the letters stand for:
(R) Set Keyboard mode to system default
(E) Terminate all tasks
(I) Kill all tasks
(S) Emergency sync the file systems
(U) Remount all filesystems read only
(B) Reboot the system
Hopefully you'll never need it - but if, it can help to prevent a lot of trouble since the filesystems comes down cleanly.