It's almost impossible to identify a problem like this. Have you done any changes at the system lately? Or is it a fresh install?. How bad is your crash? (eg. total freeze, the numlock, capslock and scrool lock lights blinks and the computer freezes or are you still able to press ctrl + alt + F1-F6 keys to access a terminal). Anyway, try going into /var/log and search for the log where you think the problem is (boot, gdm, Xfree) There might have some clues to what or where your problem is. Linux is not supposed to work like that
