You really need a schematic . . .
Either the fan motor has gone bad or for some reason it is't being turned on when needed. If you can get a schematic diagram of the wiring then it should't be too hard to diagnose, assuming that it's not controlled by an IC chip.
Have you wiggled the fan itself to see if there is excessive play? Motors can fail in more than one fashion, but often the failure mode is through bushing failure, which can usually be felt. This would also lead to a noisy motor before it quit. I've had to do a freezer fan motor in my old Frigidaire for just this reason. It never quit, but got so noisy I knew there was an issue.
Have the freezer coils iced over? That would indicate the refrigeration system is operating normally but with the lack of airflow over the coil it gets too cold and therefore ices up. If there is no ice and the motor doesn't have excessive play then I'd suspect the control circuity, possibly the defrost timer, thermostat, etc. I would guess that the fan should cycle whenever the compressor does.
Good luck!