Assuming the door is a slider one side of the door fell off the track, you might be able to lift and maneuver it back on the track....try that first, while lifting push / lifting up, pull the top edge of the door to one side, then the other...Google "sliding door hardware " images so you can get some idea as to what you are trying to engage...as long as nothing broke you should be able to reinstall..that being said if something broke you will need to replace, remove the door fully and examine the top ...usually the parts that can break are on the door....the part attached to the door jam head is typically just an extruded track...most home centers will have replacement wheels and clips...lastly if you need more access to reinstall, you might have to remove the top trim and jam...these pieces will let you see and engage the connection easier...good luck....

GENECOP is probably right, sound like the door came out of the top track at one end. There may also be a very short track at the bottom of the door that basically just holds it in place while sliding and in storage position. If the door track hardware looks good, the there is a possibility that the track pulled loose from the ceiling rafter, this has been an issue with DRV's for several years because they do not use long enough screws. If that happened, you may be able to access it from the cold air return duct and insert longer / larger screws. Pictures of the door position and track area would help. This is something you can probably fix yourself once you find the actual issue.