Diagnosis
Usually a dead halogen bulb. Could also be a faulty socket or a loose connection.
How to Fix It
Bulb is 40-watt halogen, part 12001557. Let oven cool. Press bulb holder clip inward and turn counterclockwise to remove. Pull bulb straight out from socket. Insert new bulb. Do not touch glass with bare fingers. Reinstall holder.
If That Didn't Work
Check for voltage at the socket pins with oven light switch on. No voltage means a failed switch or a break in the wire to the main control.
Heads Up
The holder gets very hot during use. Wait for full cool down. Oils from skin on bulb glass cause hot spots and premature failure.