Comment 1 for bug 152050

After you reboot the machine, look for /var/log/Xorg.0.log.old, and post that.

How completely does the machine crash?
Anything on screen - backlight on (Is it a laptop?) Does numlock work?

If you have access to another machine, does your crashed machine repond to ping?
Does it crash every time?
Do you see the screensaver come up at all?

Thats just a few pointers to get started, to see if that leads anywhere.