The license plate light is out. I changed the bulb, but it's still not working.
Sounds like you might be having an electrical issue. The circuit is simple as compared to other systems on your car, but a wiring schematic will still be necessary to trace the power distribution. There is a possibility that the bulb socket could be making bad contact as well. This is common and requires the socket to be replaced. A qualified mechanic can check the power and ground circuits of the light and make sure that the socket is making good contact. Consider the services of YourMechanic to help inspect your lights issue and make the necessary repairs.