Unit probably low on refrigerant and the compressor got too hot and shut down.

I recently bought a used Frigidaire Freezer. I brought it home and cleaned it up on the inside. When I plugged it in it ran fine and began to cool down. In a few hours it stopped running. I unplugged it and let it sit for a long while. When I plugged in back in it still did not start. I can hear a very low hum in the compressor area but you have to listed close to hear the hum. Can you give me some advice as to what the problem may be.