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First make sure the fan in the freezer is running when the door is closed.
IF OK
Unplug the unit and remove the cover so you can see the evaporator coils.
If they are heavily iced up you probably have a defrost problem.
Possible causes are defrost heater (looks like this is the glass type, these have a history of burning out may even be able to see it), defrost thermostat or defrost timer (looks like it is at the bottom of the fridge behind the kick plate). If you need help troubleshooting further just reply back with what you find.

A clue to a low freon charge is often: only frosting up on the evaporator coils is where the compressed freon enters the coils.

The below will help you troubleshoot
http://www.applianceaid.com/refrigerator.html