Diagnosis
The evaporator coils are likely frozen over, which blocks airflow. This happens if the defrost system fails. The heater, defrost thermostat, or main control board stops melting the ice that naturally forms on the coils.
How to Fix It
Unplug the fridge for at least 24 hours. Prop the doors open and put towels down. You need a full manual defrost. Once all ice is gone, plug it back in. If it cools normally for a few weeks then fails again, the defrost system has a faulty part.
If That Didn't Work
After a manual defrost, if it still doesn’t cool, listen closely inside the freezer. If you don’t hear the evaporator fan running (a gentle whirring), that fan might be dead, which also kills cooling. See the fix for the humming noise to check that fan.
Heads Up
Don’t try to speed up defrosting by chipping ice with a knife or screwdriver. You will almost certainly puncture the evaporator coils, which is a catastrophic (and expensive) repair requiring a full system evacuation and recharge.