how can I fix my APM 6321 refrigerator?

Check the thermostat. mechanical types, you should be able to hear them click, electronic, listen for compressor running or not when adjusting. No change, thermostat fault, Unless you have damaged the cooling pipes, as in scraping when defrosting, the unit should not loose refrigerant, which is the only other option. As the compressor runs, most likely fault is the thermostat.

SD
November 2011