Upright freezer door left ajar for several hours. Food defrosted, light on but compressor not working

Unplug it and remove food and put it in a safe place. Let it sit for 6-8 hours with door open so that it defrosts totally. Then plug back in and when cold put food back in.

If the compressor ran that long, it could have overheated the compressor and the high temp overload failed. They are typically a small snap switch attached to the side of the compressor and have 2 wires connected to them. As a test, connect the 2 wires together or check for 120 volts across the switch.

If you can tell us the type of freezer you have along with the model number we can find out more about it. It could also have a problem like a bad thermostat, but without checking it with a meter you cannot know what is really wrong. All anyone can do at this point is to guess at it and you cannot fix something with guesses. If it has a set of condenser coils with a fan it could need cleaning before you start trying to diagnose it. You always start with a clean unit.