I have an ancient house with ancient pocket doors, so this may not help, but one of my kids shoved the door hard and pushed it off the tracks. I tried doing what Steve suggested for about three years and finally asked my ancient carpenter for help. He cut a small square from the plaster wall--close to the floor (we no longer have the molding that should go there as it was lost in a fire) and next to the back corner of the door. He pried the door up and back on track and patched the hole (and repaired the electrical wire he cut when cutting the drywall). It works great.