The good old handy shortcut Ctrl+Alt+Backspace for restarting X Window has been disabled since Ubuntu 9.04 Jaunty Jackalope (reference). The reason is preventing Windows users from accidentally reboot his/her desktop. Sound reasonable for me.
By the way, sometimes X Window fails to load and you need something to fix it quickly. After little investigation, I found a new key-combo for restarting X in recent Ubuntu releases: