Belt came off

The belt on my 1989 astro van has come of.There was a clicking sound from the engine before but it just sounded like the fan was rubbing. Then somthing happened and when I looked the belt was no longer attached. My question is I can probably put a new belt on but. The car still starts but not one of the pullys that the belt attaches to moves. Shouldn't at least one of them be spinning to make the others move and does this mean theres a bigger problem? Can some one give me some advise?

If the engine starts and runs, the very bottom sheave (on the harmonic balancer) should be turning. It is the only one that drives the belt. If it is not turning, something very wrong has happened to the crank snout or the balancer.

it is not turning, is this somthing I could take apart myself and maybe fix or replace or is this a very serious problem?