Use the Google part of the Search function (up in one of the blue bars above) and you will find lots of posts on similar problems.
It often occurs because of an accident, say the door flies open while on the highway and gives itself a huge whack, or perhaps someone slammed it shut with an item wedged in the top.
In any event, the repairs seem to follow one of two main methods:
- Arrange some combination of wood (2"X4" perhaps) and force (shoving) on the door to force it back into shape, or
- Remove skin(s) (inside and/or outside)then adjust tension of same, and finally re-rivet them on in a manner to have the door "hold" a revised shape.
Good luck with it, it's not a job I'd like to have to do.