Whirlpool upright freezer AFG823NF-Thermostat not working properly

JennyT just trying to help with this. I assume this is a self defrost model. If so then is there an ice build up on the back wall? If so then the unit isn't defrosting. That is why pushing the Fast Freeze button is no longer working. You can get by however. Remove all the food from the freezer, put a vacumn in the freezer and shut the door. Turn on the vacumn, this will pull the warm air thru the freezer and in 30 minutes to an hour it will be defrosted enough to work until you can get someone to replace the 1 of 3 parts that the defrost system uses. The vacumn is what I use to defrost my nonself defrosting freezer. Sorry I couldn't be of more help.