How to fix stuck pocket doors?
This type of door is called a pocket door.

The first possibility is that the door has simply come off the track or the door has come unattached from the hanger that runs along the track.  In that case you can figure out which side of the track it should be hung from and pick the door up and rehang it or attempt to fish the hanger out of the pocket and reattach it to the top of the door.

The more likely case, in my experience, is that the hardware is broken and will have to be replaced.  Here is a good guide to replacing the hardware on a pocket door: http://www.ronhazelton.com/projects/how_to_repair_and_replace_a_pocket_door