It sounds like X is crashing.
Check your log files after it happens, that should yield something to put in Google.
Also, instead of 'Ctrl + Alt + Del', try 'Ctrl + Alt + Backspace' to restart X.
If this doesn't work, try 'Ctrl + Alt + F1', login when prompted and run one of the following:
$ sudo /etc/init.d/gdm restart
$ sudo /etc/init.d/kdm restart
Hope this helps.