Starting from release 11.10, Control Alt Backspace key combination in Ubuntu doesn't work by default; it's disabled. That means you cannot kill or restart X server with Ctrl+Alt+Backspace if something goes wrong with the user interface (e.g. screen freezes from some reason).
Luckily, you can turn it on again following the steps below:
1) Go to System settings (gear icon in the top right corner) > System Settings...
2) Click Keyboard Layout icon
3) Click the Options button in the bottom right corner
4) On new window you'll find an option Key Sequence to kill X Server
5) Mark the Control + Alt + Backspace checkbox, close the window and you are now allowed to restart X server and your session on Ubuntu 12.04