Diagnosis
The halogen bulb is burned out or the door switch that triggers the light is faulty.
How to Fix It
Replace the bulb first. It's a standard 20W Halogen T4 (JCD Type) bulb. Access it by removing the bulb cover inside the microwave cavity.
If That Didn't Work
The door switch assembly has multiple micro-switches. The one for the light may be failed. This requires a door switch replacement.
Heads Up
The bulb gets extremely hot during operation. Let the microwave cool completely before replacement. Do not touch the new bulb with bare fingers.