DIAGNOSIS: The air damper that controls cold air flow from the freezer to the fridge is stuck closed, or the evaporator fan in the freezer has failed.
FIX: Listen for a fan humming in the freezer (back wall). If silent, the evaporator fan is dead and needs replacement. If you hear the fan, locate the air duct/damper between the compartments (often behind a panel in the back of the fridge section) and check for ice blockage.
IF THAT DIDN'T WORK: A sealed-system issue, like low refrigerant or a clogged capillary tube, could be the cause. This requires a professional LG technician with proper tools to diagnose and repair.
HEADS UP: Don't just crank the fridge temp setting to max. If the damper is stuck or fan is out, this won't help and will just overwork the freezer. Diagnose the airflow first.