DIAGNOSIS The door switch is faulty or the bulb is burned out. The door switch is a small plunger-activated button near the top of the door frame that turns the light on and off.
FIX Unplug the fridge. Use a flathead screwdriver to carefully pry out the light cover/lens inside the fridge. Check the bulb first. If the bulb is good, locate the door switch plunger. Press and release it manually. If it feels mushy or doesn't click, the switch is bad. The switch is held by clips or screws and has wire connectors.
IF THAT DIDN'T WORK The wiring harness from the switch to the main control board may have a break. You'll need a multimeter to check for continuity from the switch terminals back to the board connector.
HEADS UP When replacing the bulb, use only the exact wattage and base type specified. An incandescent bulb that's too hot can melt the plastic housing.