Diagnosis
The bulb is burned out, or the socket/connection has failed. This model uses a specific halogen bulb, not a standard household bulb.
How to Fix It
First, try replacing the bulb. Unplug the microwave. The bulb cover is inside the cavity, usually on the top right or left wall. It may twist off or have a screw. Carefully remove the old bulb and replace it with a new 25-watt halogen bulb with a T4 base (often labeled as part #WB-251 or equivalent). Do not touch the glass of the new bulb with bare fingers; oils can cause hot spots and shorten its life.
If That Didn't Work
If the new bulb doesn't light, the issue is in the wiring or the door switch that activates the light. The same door interlock switches that enable cooking also often control the light circuit. A faulty switch could be killing power to the socket. You'd need to test the switches as described in the door interlock fix.
Heads Up
Never use a higher-wattage bulb or an LED not rated for microwave oven use. The heat and microwave energy can cause a standard LED to fail or even be a safety hazard. Stick with the exact halogen replacement.