if the latch lines up and the bottom of the door does not drag
only binding, at the top
could be the roof/timbers above the door that are sagging and putting roof weight on the door frame
if this is the case, then its opening the wall and possibly the roof in order to do a repair
or maybe remove the door, and 'maybe' adjust the frame or trim the members above the door frame to make it square
depends on the construction in the wall, alu tubing or wood
tubing is stronger, wood is easier for a DIY repair
might need to leave be and not use it, wait for warm dry weather ?
somebody with more experince needs to inspect it and give you a professional opinion
its not beyond DIY repair, but you need to know what you are getting into and have a repair plan and good weather, or indoor storage
