Jeep Repair: Serpentine Belt wheel came off, serpentine belt, water pump

Question
Last night one of the wheels that my serpentine belt goes around came off.  It does not have ridges, it is smooth and the back side of the belt runs around this wheel.  I think its the power steeing wheel, but it could be the water pump.  A small shaft comes out of the engine block that the wheel connects too.  I still have the wheel.  How does it attach?  Do i need a special part to reconnect it?  Thanks!  My jeep is a 1994 Cherokee with an inline 6 cylinder, 4 liter.

Answer
does the shaft have a hole with threads that u can put a bolt into it?  If so u can get a bolt to fit it  and u will more then likely need a new pulley because i can bet that the spacer for it is gone and that is needed in order for it to spin freely.