What Caused Strange Hardlock?

Heard from my best friend yesterday and he had experienced a strange hardlock recently with his system. After locking his system wouldn't come back up after a warm boot or a cold boot. He had to clear CMOS to get it functional again. Any ideas as to what could cause this? System specs follow:
Abit KX7-333
XP2100
512Mb Samsung PC2700
GeForce4 4200 128Mb
Philips Rhythmic Edge
Generic 400W PSU
Windows 98SE

TIA...
Tomac