Once you get a good frost pattern on the evaporator, run a manual defrost again and watch to see if the heater gets hot and melts the frost. If it does, you need a new electronic defrost timer. If not, check the limit and heater as previously suggested to determine which one has failed. It will take a couple minutes for it to start defrosting once manual defrost is started. It should have thawed it all out within half an hour. Hope this helps!