nothing is indesructible and a machine can lock up completely. It is just less likely to happen than in some other OSs. Very often, when it seems to have crashed, the machine is still running. Try pressing ctrl-alt-Fn (F1 thru F6). If it's still running, you'll get a text login screen. You can login and do some repairs from there. If X crashed, you could restart it by
startx
or by becoming root and then
init 3
and then
init 5
which brings back the graphical login.