When you've defrosted everything and restarted the fridge/freezer, can you hear the evaporator fan running? It only runs with the doors closed. If you keep your ear to the unit you may be able to hear it stop and start as you open and close the door. Do not confuse the sound with the compressor motor fan which runs when the compressor is running.
If you can't hear it running at all, either the fan is faulty or the fan controller circuit is faulty. If you're quick enough and you can't hear it above other noise and you have the back panel and fan panel off you may be able to see the fan spinning down when you open the door. (Obviously the fridge has to be switched on - see safety notice)
If you can hear it running then possibly the defrost heater may be faulty.
Not sure about your model but on some fridges there is an electrical diagram (usually on the back) which may be of some help - if you can read wiring diagrams that is.
Safety Notice: Unless you know exactly what your doing always work on electrical appliances with the power disconnected and unplugged and even then be aware.