I assume that you have looked at all the other causes for the unit not cooling. If not, here are some things to checks. Are condenser coils clean? Is the condenser fan running? Do the evaporator coils have ice built up on them? (indicating that the autodefrost is not working). Is the evaporator fan running? Do the lights shut off in the fridge and freezer when the doors are shut (the heat they put out can consume half the unit's refrigeration capacity). If all that checks out then youhave ot think about refrigerant charge level or the health of the compressor.