Well, either one of two things is happening here...
1. Your roomie cracked the core when he was putting the fancy new heatsink on...
or
2. The motherboard is shutting down because there is no fan plugged into the "CPU Fan" header of the motherboard. Connect a fan (any fan with a tach output) to the fan header that the cpu fan was plugged into and try again.
Hope this helps... I'm betting on #2