Removed the panel at back of freezer, found that the evaporator coils were completely iced over. Removed the connectors at the of the defrost heating element, it was OK, had continuity. Decided that the problem was either the defrost thermostat or defrost timer. Ordered both parts, as they are cheap and testing the thermostat can be difficult.
Also, used a hair dryer to defrost the evaporator, which allowed the refrigerator to start cooling both sections, at least for a few days, until new parts arrive. It will work until the evaporator coils frost up again.
When installing the defrost thermostat, I soldered the connections and covered with shrink tubing and put silicone at the ends.
New parts arrived and replaced both parts and refrigerator is working.