I accidentally did the same thing last week with a customer's computer (heh 00ps..), the heatsink was hot enough to cook bacon on and the system shut itself down. However I let the heatsink cool down, plugged the fan back in, and everything was okay from then on as if nothing had happened. This was with an Athlon XP 2400+ (T-bred).
Let it cool off for 20 minutes or so, PLUG THE FAN IN, and power it up. Most modern CPUs shut themselves off as temps get high enough to possibly cause physical damage, so might be okay.
Which begs the question... what processor were/are you using?