Take 1 wheel nut from any wheel, or if you have a spare one then use that.
Put the nut in between the lower door hinge, like so.

Now slowly close the door with some light force. Don't go overboard and whack it closed. That force should bend the door back up to level as it should be. Do it with little force many times, you don't want to over do it, but if you do then do the same on the top hinge, and the door will drop down again.
I had to do it 6 or 7 times before I was happy with the result, just make sure you do it in small steps checking if you are happy with the level of the door as you go along.