Hard lock. Where the thing just locks at whatever it was doing and stops accepting input. Monitor continues to output video, but nothing is animated of course.
It's a video card freeze.
Are your video cards overclocked or undervolted?
If it happens again, try to ssh from another box if you can.
Have to hold down the power button to reset.
Didn't you try the magic sysrq keys (in Linux, of course)?
Sync first (S), then remount ro (U), then reboot (B).
http://en.wikipedia.org/wiki/Magic_SysRq_key--
Bye