My frigidaire freezer is not working an not cooling, the fan is running.

Have you checked the temperature settings? Or refrigerant levels?

Unplug it and pull off the back. Check the condenser coils. Clean with spray soap and wash off with water hose nozzle then vacuum up water with wet-vac. Make sure to clean the fan blades, too. A little dirt on the fan blades will keep it from blowing enough air. Replace back, but check front kick plate and clean it, too. Remove it and wash it off. See how it works. Unless you have had a broken line there is no reason to check your "freon" levels. That's the last thing you need to worry about. Make sure the compressor is coming on, though. If not, see if it's hot. If so, keep turned off until it cools down as dirty coils will cause it to overheat and shut down.