If the unit defrosted itself then likely the control thermostat or compressor is the problem.
The control thermostat regulates the temperature of the unit. If it cycles off but then doesn't come back on then things can defrost. If the unit is not running then try tapping the thermostat or turning it off and the on and see if it starts to run.
Another possibility is the compressor over heated and shut down on overload. It might stay off for a few hours till it cools down and starts to work again.
If the unit is working then you might check and see how hot the compressor is. If you can't keep your fingers on the compressor because it is to hot then the compressor might be the problem.
Compressor amp draw can be checked with a clamp on meter. Anything over 5 amps would be suspect.
If the unit is defrosting itself then listen for a click from the compressor when it tries to run. The click would be the overload tripping due to compressor overheating.
I hope this helps. A freezer is not something you want to have this problem with.
You can reply with any additional details or questions for further assistance.