One thing I neglected to mention is you could also have a bad evaporator fan or it could be blocked by ice build-up. If you fridge has warmed up, it would not be ice blockage.
You can remove the evaporator cover on the back inside of the freezer and measure the voltage of the wires going into the fan motor. If the thermostat / temp control and defrost timer are working properly, you should measure 110V at the connector going into the fan motor when the temperature is turned to the coldest setting and calling for cooling. If you have 110V when it is calling for cooling but the fan is not moving, the motor is bad.
__________________
|