You should try to find out if the system has totally hung, if it's just X that has hung or if it's something else.
Try key sequence:
Ctrl + ALT + Backspace
This sould kill/restart the X Server.
This switches to the terminal. If you get a terminal, then it is likely that you're having troubles with X. You can view the log which is normally located here: /var/log/Xorg.0.log
You'll need to post what hardware you're using in order for anybody to help you further. It could be a driver problem.