A belt in my truck has broken. Help? on 1990 GMC 2500 Pickup

This morning I walked outside towards my truck and I saw that there was a belt laying on the ground broken underneath the truck by my tires. This is the first time it has ever happened. Is this a major $ problem to fix? What could possibly be wrong? It was running great as of late.

Asked by for the 1990 GMC 2500 Pickup