Sounds like the actuator (the part that turns an electrical signal into moving locking / unlocking your door) is stuck.

To get access to it you will need to remove the trim from the door. Usually the unit itself is held in with a couple of bolts that can be accessed from the rear-facing end of the door.

Depending on the car it can be very easy or very hard to get the actuator out, so your mileage may vary. At the very least you will have to disconnect all the mechanical linkage (door handles and such). Pay very close attention to what goes where and how it is oriented.

Once you have it out, try lubricating it (this can sometimes get them going again). I would not recommend WD40 for this, instead use a PTFE-based lubricant (which also comes in a spraycan). Spray it on and in every moving part you can access. If there's a rubber boot, lift that up and spray inside of it as well. Don't forget to put the boot back. Let it air out for a bit (or drip out if you were very generous with the spray ;) so the solvents can evaporate.

Then just hook it up to the harness and test, if it works put everything back in place. If not, replace the unit. If you want to be sure you can complete the repair in one go, order a new / second-hand actuator beforehand.