DIAGNOSIS The door-activated light switch has failed. It's one of the three door interlock switches. That specific switch only controls the light circuit.
FIX Unplug the unit. Remove the vent grille. Identify the three door switches. The light switch is usually the one with a different colored actuator or wiring. Test it for continuity. Replace the faulty switch.
IF THAT DIDN'T WORK The wiring harness between the switch and the main board may have a break. Visually inspect the wires for a pinch or cut near the door hinge area.
HEADS UP The bulb is a specialty Halogen JCD type. Do not touch the glass with bare fingers when replacing. Use a cloth.