OK, if the fan in the freezer compartment is running, but there's no draft of cold air in the freezer compartment, that would suggest to me that the airflow over the evaporator coils is blocked, perhaps by ice.
When you open the freezer door, it's the "evaporator fan" that you hear running and that creates the cold breeze in the freezer compartment. If you say that fan is running, but you don't feel any cold breeze, then something has to be blocking the air flow. What's crossing my mind is that your defrost heater isn't working, and the frost accumulation on the evaporator has grown so thick that you no longer have proper air flow over the evaporator coils.
If you look in your freezer compartment, you should be able to see a removable panel held in with screws. There may be plastic caps over those screws. Can you see such a panel?