Begin by setting the auto door locks of the vehicle to their locked and unlocked positions. Listen closely for a buzzing noise and a vibration coming from any of the locks that don't function. If this noise is present, then a door lock actuator has burnt out. These are small motors placed in the door panels that lock and unlock the door. They can be replaced without great difficulty, though getting the door panel off can be difficult depending on the model car.
Should the auto door locks not function and not make any noise, use a multimeter to check the car's fuses. A burnt fuse may look no different than a functional one, but it may prevent auto door locks from working correctly. Also make sure that the wires running from the fuse box prior to the point where they split and head for each of the door panels are unbroken, with no signs of charring. If the wire prior to the point where it splits into multiple smaller wires is damaged, none of the door locks would be receiving power to function.
Should only one or two of the door locks fail to function, the problem is likely within the door panel rather than at the fuse box. Open up the door panel or panels of the locks that aren't working correctly. Trace the wires running from the door lock actuator to the power relay (which will look like a square black box and sit next to the lock/unlock switch on the door), out to the fuse box. The wires should be undamaged with no soot or charring. Test each wire at either side of the relay box. Should there be power in the wire running from the fuse box to the relay, but no power in the wire running from the relay to the door lock actuator, the relay is broken and needs to be replaced.
