Hey guys...the serpentine belt on my 96' sonoma broke today, so i went to napa auto and got a new one, and proceeded to install it. I was able to pull the tensioner back to get the belt on, and everything looked good, but once we started the car, smoke came from the belt, and it appears one of the pulley's is siezed. The culprit is the metal pulley "thing" that is right in the center of the engine that has a shaft coming out of it that goes to the cooling fan. We tried starting it again to see if it might loosen itself, but no luck. What needs to be done to get this part loose again? Can i do it myself or should i take it to the family mechanic?

Also, when the serpentine belt broke (my brother was driving the car, i was not there) he said the motor still ran for like 30 seconds. Does this mean that in that 30 seconds, with no serpentine belt, on this truck will valves collapse or other harmful things happen inside the engine? I am not too familiar with GMC's or american cars for that matter, I know Hondas=/. Any help you guys can offer will be appreciated!

That metal thing attached to the fan is the water pump.
You will need to replace the pump.
Should not have caused any damage if the engine did not overheat.