If the screen and input devices (keyboard and mouse or trackpad) froze, the first place to start by looking would be in `/var/log/Xorg.0.log` (assuming that Xorg is running on the first display server).
If that doesn't yield any immediate clues, the next logs to check would be `/var/log/messages.log` and `/var/log/dmesg.log`.
If you are unable to find anything in the logs, and the freeze is happening with any frequency, you might be advised to check your memory with a utility like memtest86+.