Check the track to make sure it is not bent in anyway. If the track is bent or has opened up too wide the wheels can fall out of the track. Some doors can be lifted up and set back in the track if they only fell off the track. If that does not work you may have to remove trim around the doorjamb in order to swing the door back onto the track. There are different types of tracks out there. If these ideas do not work you may need to contact a local professional. You can visit www.servicelive.com
to find a local professional. Good luck.