The fan may have stopped working due to the build up of ice on the evaporator coil in the freezer. The most likely cause would either be a burnt out defrost heater or a bad defrost timer. Normally the defrost timer turns the heater on for about 30 minutes every 8 hours to melt the ice on the coil. If the timer fails it will either stay in cooling mode or defrost mode. The timer is usually located in the temperature control box in the refrigerator. It looks something like this:
On the side of the timer is a small cam that you can turn with a flat screwdriver. With the refrigerator plugged in, turn the cam until it clicks. When it clicks, it should be in defrost mode and the heater should start to get warm and melt the ice. You should be able to hear the ice crackling and hissing if the heater is working. If it does nothing then the problem is probally due to a burn out heater.
To get to the heater, you will need to remove the back panel from the freezer and the heater will be attached to the coil. You will need a hair dryer to melt the ice from the coil, to get to the heater. Parts can be found at your local appliance supply house or online at repairclinic.com. Hope this helps you.