Diagnosis
The defrost system has failed. The heater, defrost thermostat, or defrost timer (if equipped) is not activating to melt the frost on the evaporator coils.
How to Fix It
Unplug the unit. Remove all food. Let it thaw completely for at least 24 hours. This is a manual defrost. Once all ice is gone, dry the interior, plug it back in, and monitor. If it frosts over again quickly, the defrost component has failed.
If That Didn't Work
The defrost heater or bi-metal thermostat is faulty. These are not user-serviceable parts in this compact design. The control board may also be the culprit. Service is required.
Heads Up
Do not chip away ice with a knife or screwdriver. You will almost certainly puncture the evaporator coil, which contains refrigerant. The unit will be unusable.