Why did my serpentine belt snap?

I have a 96 s-10 4 cyl 2 wheel drive, and the belt snapped. the pulley that has the radiator fan on it seems to be frozen. what is this pulley, the water pump? any ideas of what could cause the pulley to lock up, is there any way to get it moving again without buying a whole new water pump and pulley? if im not asking the right questions what is the info that i need to get this job on the road. thanks any ideas would be appreciated.