Ok the problem here is going to be with either the defrost heater or defrost thermostat, Open the freezer and take the shelfs out, then remove the lower panel inside the rear, If this is a top mount freezer remove the panel in the rear.
Once the panel is out you will see the evaporator, Thaw the frost on the evaporator with a hairdryer or let the unit sit overnight with the door open. Once the ice is thawed you will see the defrost heater looped below the evaporator. Test this with a ohms meter, if it reads open then it will need to be replaced.
If the heater reads good (continuity) then the defrost thermostat is bad and will need to be replaced. This is located clipped to the top corner of the evaporator and looks like this.
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