Pocket doors are almost exactly like a closet door. They are suspended from rollers at the top of the door onto tracks mounted at the top of the pocket doors frame. If they come off the track they just need to be lifted back up on them like you would a closet door. The big problem comes into play when they come off track while inside their pocket. This is when some good old brute force comes in handy. Use somethoing like a chisel on top of a hammer handle to pry up with using your foot to push down on the chisel with. Wit your free hands try and guide the roller wheel of the door back onto the rail. Once you get one or the other set you can apply opposite force to the door and slide it out enough to get a good grasp on it and set the second roller.
As a last resort and for something to grab onto I have inserted 3" drywall screws into the ends of the door and hooked my framing hammer to them to pull with. I have even ran a series of opposing screws in either side of the door and worked one out like that. It was wedged in so hard that the pocket frame gouged the door getting it out.
Please post any other concerns you may have here.
Best regards, XXXXX XXXXX Smithy