Diagnosis
The evaporator fan in the freezer has failed, or the air damper that lets cold air into the fridge is stuck closed. The fan is the most common failure.
How to Fix It
Check the evaporator fan in the freezer.
1. Unplug the fridge. Remove all items and the rear panel inside the freezer compartment.
2. Locate the fan (usually near the top back, behind the coils). Manually spin the fan blades. They should move freely.
3. Plug the fridge back in briefly (be careful). Listen for the fan to run when the compressor is on. If it doesn't spin, the fan motor is dead and needs replacement.
If That Didn't Work
The damper control (a flap that opens and closes between the freezer and fridge) is faulty. It's located behind the vent cover on the back wall of the fridge compartment. If it's stuck shut, no cold air flows. You can try gently moving the flap to see if it's free, but the damper actuator motor often fails.
Heads Up
Before you start, ensure the vents between the freezer and fridge aren't blocked by food packages. A simple blockage can cause this exact symptom.