I had this problem with a 1984 vintage GE fridge with freezer on top. The
defroster used a halogen lamp that had burned out , the defrost cycle
wouldn't happen, and the passage for air between the freezer and fridge
compartment froze up not allowing cold air to pass.
Also had a similar problem with a Whirlpool model of same vintage at the
in-laws summer home. The fan that forced air from the freezer to fridge
stopped turning due to lack of bearing lubrication.
Either way everything has to come out, unplug from wall, the unit manually
defrosted, water removed, then you take out the inside back panel in the
freezer to reveal the problem, get the parts (GE parts are very overpriced),
or relube the fan, put it all back together again and perform smoke test.
Current unit is a 1988 vintage Amana. No problems ever.