In addition to the PID story in Run-Away Processes If all is blocked and even Ctrl+Alt+backspace ( what should be a last resort ) does not react, if your system does not react on any action ( like I said before, very rare in
Linux ) remember the next line:
Raising Skinny Elephants Is Utterly Boring
Here is how you "raise the elephant":
Alt+SysRq+r ( The LEFT Alt key ) ( SysRq is on the same button as print screen )
Alt+SysRq+s
Alt+SysRq+e
Alt+SysRq+i
Alt+SysRq+u
Alt+SysRq+b
Give a little time between keystrokes.
The r stands for put keyboard in raw mode
The s for sync the disk
The e for terminate all processes
The i for kill all processes
The u for remount all file systems read only
The b for reboot the system
THIS IS THE VERY LAST SAVE YOUR BUTT PROCEDURE ONLY IF ALL ELSE FAILS !