Right rear door in my 2011 GMC Terrain will not open from the inside or outside. The latch seems to be jammed. Help!

Do you know why it was difficult to close before it got stuck? Was the bolts holding the door on to the frame come lose or out of adjustment? Will the latching mechanism unlock the door so it can be opened, even though ti appears stuck? Have you tried a long bar to push the door back towards the frame and at the same time using a strong shoulder or sledge hammer (not recommended) to push open the door? From the inside, try to pry up on the door to see if it moves. Use a steel flat nosed pry bar to place a lot of weight on the door itself to see if it will at least move up and down, or even back towards the frame and open the door. Also have you been in a collision so the frame is possibly bent? Something came lose and is now jammed in tight. Perhaps the locking mechanism will not release due to the miss adjustment. Worse case would be to find a used door in a pick your parts yard and yank out the door and replace it.