Sometimes it's just the X server that is frozen. Try to get a console by hitting CTRL-ALT-F1. If that won't do, try logging it your machine using SSH (install openssh-server package first). If you can't get a shell, reboot.
Once you have a shell, check the system logs (/var/log/syslog, /var/log/messages, /var/log/Xorg.0.log and ~/.xsession-errors). Scroll up to the time the crash happened. If you didn't have to reboot, the dmesg command will show you the kernel log buffer in case it can't write to your filesystem.
Whether or not you are able to find anything in the log file, what makes it easier to debug is being able to reproduce the problem at will. If you can't, and the logs show nothing unusual, then you can try to isolate the source of the problem by booting a rescue CD (or a completely different OS), removing non-essential pieces of hardware, or replacing some parts with others that are known to be functionnal. Doing this can be tedious, and requires rigour.
Once you have minimal information, file a bug report for the suspected faulty software, unless you think your hardware is faulty.
Sometimes it's just the X server that is frozen. Try to get a console by hitting
CTRL-ALT-F1. If that won't do, try logging it your machine using SSH (install openssh-server package first). If you can't get a shell, reboot.Once you have a shell, check the system logs (
/var/log/syslog,/var/log/messages,/var/log/Xorg.0.logand~/.xsession-errors). Scroll up to the time the crash happened. If you didn't have to reboot, thedmesgcommand will show you the kernel log buffer in case it can't write to your filesystem.Whether or not you are able to find anything in the log file, what makes it easier to debug is being able to reproduce the problem at will. If you can't, and the logs show nothing unusual, then you can try to isolate the source of the problem by booting a rescue CD (or a completely different OS), removing non-essential pieces of hardware, or replacing some parts with others that are known to be functionnal. Doing this can be tedious, and requires rigour.
Once you have minimal information, file a bug report for the suspected faulty software, unless you think your hardware is faulty.