Freezer not cooling

I have a side by side Frigidaire refridgerator which is not staying cold. I did not know anything about refridgerators until today. After doing some research I have determined the compressor is working and the condensor fan is working; however, the condensor is not getting cold. It is barely below room temperature. I had some dust on it and I cleaned it off but it does not seem to have helped. Does anybody have any ideas on what the problem is?