By any chance did you put a LED light in the courtesy light? If so, the door courtesy light housing has an internal diode in it. If your LED is polarity sensitive switching the LED around may fix the problem.
If you are still using a conventional bulb and you know the bulb is good, then you likely have a door wiring problem (the light may have become unplugged) or an internal failure on the socket or the internal biasing diode may have failed. If the dome light is still working that is good because it indicates the latch switch is still operating (a double hassle and expensive to fix). Unfortunately, to remove the courtesy light to check the socket or look for wiring problems requires that you remove the door interior panel. Get a hold of the service manual before you do that because you can break expensive things if you are clumsy when removing the interior panel.