A make and model may be helpful!!!!
You say the fan is running. Which fan.
They usually have a minimum of two.
One in the freezer/evaporator fan.
And one in the machine compartment condenser/compressor fan.
Is the compressor running you can usually hear it.
It should also be vibrating a bit and warm when running.
If the condenser/compressor fan is running then the compressor should also be running.
If the compressor is running and you get no cooling at all in the freezer then it is probably a sealed system problem and not worth fixing.
If it is an electronic controlled unit then try unplugging it for a couple minutes. This may reset the computer.