When your car's automatic door locks are not working, there are a number of potential causes for this, some of which are actually very quick and affordable fixes. Note a few common reasons for that lock to fail so you can check these yourself, or know what your mechanic might need to address to get them working again quickly.
An actuator is a type of electronic component in a car door that controls remote power locks; when you use the transponder on your key ring to engage or disengage the lock, this actuator conducts electricity from the car's battery and sends commands from the car's computer to the lock. If the actuator is worn out, the computer system and the door lock won't get power or communicate with each other, so the lock won't engage or disengage. This actuator is a somewhat simple piece for an automotive electrician to replace if it's worn down or malfunctioning.
When you use the transponder on your key ring, this sends a signal to the car's computer which, in turn, sends that signal through the actuator to make the lock engage or disengage. Like your computer at home, your car's computer can malfunction so that it's not sending the right signals to the lock. One way to test this is to unhook the car's battery and let it sit for half an hour or so, and then reattach it. This should reboot the car's computer, just like rebooting your home computer. This can fix any malfunctioning programs in that computer so it can communicate with the door lock once again.
Bad transponder chip
As with the computer in your car, the transponder on your key ring works with a small chip that lets it communicate with the car. If this chip goes bad, the car is not getting a signal that it should engage or disengage the lock. Your car's dealership or an automotive electrician can check the transponder and note if it's working properly, and then replace that chip if needed.
Broken wire in hinge
The car's computer communicates with the actuator mentioned above through wires that run through the hinged area of the door. If one of these wires is broken or frayed, the door lock is not getting the right signals or enough power to work. If you've recently had work done on the door or have been involved in a collision that damaged this area of the car, there is a good chance that this wiring was also damaged and simply needs replacing.