Food on bottom shelf and bottom basket is frozen. Food on upper shelves and door has thawed. Unit is still running. Could the fan be out?

There are three possible problems and all of them take a meter. First, find the defrost timer and rotate until unit turns off. If the heaters are okay it will begin to thaw and continue until finished. If the timer is okay it will start up again, if not it will stay off meaning you need a new timer. If this fails you need to thaw the freezer out enough by unplugging it and when it is thoroughly thawed remove the cover over the evaporator (cooling coils in freezer). You must be careful because you can easily break something. It's best unplug it and to take it outside where you can hose it out on the inside and underneath where the condenser coils are located. You need to have it totally clean to properly diagnose it. When cleaned inside and out, leave unplugged and remove the cover over the evaporator. There you will find a fan and defrost heater and thermostats for the fan and defrost heater. Test the wires leading into the motor with an ohm meter. There should be continuity across the two terminals and none to ground. It should have about 1-2 kilo-ohms of resistance, not mega-ohms. You also have a fan t-stat that is sometimes combined with the defrost t-stat. Test the continuity across the defrost heater; if open you need a new one and always replace the defrost t-stat with it. The defrost t-stat can be tested by putting it into a freezer and getting it below 0 degrees when it should be closed and read continuity. If open, you need to replace it. The fan t-stat will also close at 0 degrees and you can also test it at the same time. When you find the problem, reassemble and it should cool fine.
DrTom4444