It's possible that the defrost heater unit has gone bad or the timer that controls the heater is bad. The timer may be behind the grill below the door or behind the fridge. The one on my fridge had two curved ramps (so you can't turn it backwards). Turn it until you hear it click and wait an hour for it to defrost. Turn it again until the compressor starts and see if it freezes. The problem is that the evaporator coils get covered with ice and air cannot cool well enough for the unit to function properly.
