Fridge not cold but freezer fine: the defrost cycle is broken, not the compressor

If the freezer is frozen solid but the fresh food side is warm, the evaporator coil behind the freezer back panel is iced over and blocking airflow to the fridge. Pull everything out, remove the back panel, and you will see a block of ice. Thaw it with a hair dryer (not a heat gun, the plastic melts). Then find out why: the defrost heater (test for continuity), the defrost thermostat (should be closed when cold), or the control board / timer. Each is $15 to $40. A fridge running fine for two weeks after a manual thaw and then warming again is the signature of this problem.