Do any of the door handles inside or out feel sloppy (when you pull on them does it feel like there is nothing connected to them)?
If so that the levers on the latches are probably rusted up and sticking. Say for example the outside door handle lever that is attached to the door latch is stuck half way in the open position it will not allow the door lock mechanism to lock or unlock. The door will still latch and stay closed because the lever will not be stuck far enough to release the latch mechanism from hold on to the striker.
I have had pretty good luck removing the door latch (the part that actually holds the door closed) and soaking in penetrating lube and working all the levers until they move freely.
As far as the door that doesn't open I suspect it is stuck in the lock position and one of the handle mechanisms is in the partial up position. Again the symptom would be a sloppy door handle. That lever will need to be put back in its normal resting position before the door can be unlocked. This can be done with a slim jim. Once you have the door panel apart on the door that still opens you will be able to see the parts you need to move. You can then use that knowledge to move those parts on the other door that doesn't open.
If you don't have any experience with a lockout kit (i suspect most people do not) that that's the best way I can describe how you can do this yourself.
I’m happy to assist further over the phone at
https://www.6ya.com/expert/jeremy_d728a59f986299fa
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