When your car's door locks don't work, the problem could be electrical or something mechanical in the door lock itself. An auto locksmith can usually examine the car for either such issues, but note a few potential problems that would cause a door lock to malfunction, so you know what you might be expecting by way of repair bills.
One thing that many car owners overlook when something electrical fails is the fuse. Like the circuit breakers in your home, the fuses will pop when something gets too much electricity. You can check your owner's manual for the location of the fuse box and to discern if one has blown; these are very easy to change on your own.
Lock cylinder and assembly
Similar to the locks on a home's entryway door, the lock cylinder and assembly in your car may come out of alignment, or the cylinder may have rusted. This keeps the lock button from moving as it should. Activation rods also work the lock mechanism itself, and when one has broken or come out of place, the lock gets jammed.
The lock cylinder is hidden in the door panel and it's best to let a repairperson check for this damage, as improperly removing a door panel can cause it to actually break off, and you won't be able to close the door again. A locksmith or repairperson can safely remove the panel and check if the cylinder is rusted or the lock assembly has come out of place and if the lock itself would then need replacing.
Door lock motor
A power lock has a small motor that moves the lock cylinder and rods into place when the lock is engaged or disengaged. If this motor burns out, the key fob may be working for all other doors, but this one door will refuse to engage. This motor is usually simple enough for an auto locksmith or mechanic to replace, but it's also located in the door panel, so let a repairperson check it and replace it as needed.
The wiring is bad
As with all electrical parts, your car's door locks will have wiring connecting it to the battery and its own motor. When the wiring is bare or frayed or has come disconnected for any reason, the electrical systems of the lock won't work. This wiring is also in the door panel and runs from the door to the area under the hood of the car, but a repair person can test each section to note if it's conducting electricity properly.