Method 1. Use something to act as a brace against the door such as chair or stick.Method 2. Jam a wedge into the door at the bottom.Method 3. Install a lock or other securing device.
First remove the Door panel on the vehicle and make sure any of the lock or handle rods are not broken or diaconected. Secondly if that's not the problem you may have a broken door latch. Price...
The hook portion or the latch is probably flipped up. If you lift the exterior handle up and take a pen or reasonable facsimile, you can flip it back. Post your results.
Answer.
Should someone...