Diagnosis
It’s almost never the bulbs. Each fresh food door has a plunger-style switch that turns the lights on/off when the door opens/closes. One of those switches has failed or gotten stuck.
How to Fix It
Find the switches. They’re small rectangular buttons on the inner frame where the door closes. Press each one in with your finger—you should hear/feel a click. If one feels mushy or doesn’t click, that’s the culprit. You can order a replacement door switch (part number is often WR55X10025 or similar). It’s held in by a couple of screws and has a simple plug connection.
If That Didn't Work
If the switches click fine, the next suspect is the main control board. But before that, check the LED light strips themselves for any visible damage—though they rarely fail all at once.
Heads Up
The lights are LEDs, not user-replaceable bulbs. If a light strip is actually dead, you have to replace the whole assembly, which involves removing door liners and is a much bigger job.