On Ubuntu it used to be that ctrl-alt-backspace would kill X (the backend of the various graphical user interfaces on Linux). In an aim to be user friendly this is now disabled by default. This can be a real pain if X locks up, you can’t kill it nor change to a console.
On Debian and Ubuntu you can install the dontzap command which will allow you to kill X:
sudo apt-get install dontzap
Then run dontzap:
sudo dontzap -d
Or you can the following section to your /etc/X11/xorg.conf file (which is what the dontxzap command does):
Section "ServerFlags" Option "DontZap" "False" EndSection