It sounds to me that the nut on top of the blade arbor (top side of the deck) came off or broke off and the arbor fell down. Missing this pulley tension, it would stop the belt from turning the other two arbors.

I'll bet you find a pulley laying on top of the deck.

Wow Scary Incident ...Sure gives a reason for fastener lock-nuts , Safety Wiring or Thread Locking Sealant Products or the Very Least Proper Torquing Factors !

Thanks guys, it sounds as if the actual damage is less than I had been concerned about, except for the potential we had for a accident.

I will take off the top guard cover and see what the damage is as I have not been able to find the nut in the yard.

when looking for the missing nut, look for a spacer. the spacer is located under the pulley.

One would assume that the spline in the pulley's center could be damaged also.It is likely more than just putting it all back together.