Remember that most all refrigerators only have cooling to the freezer... the refrigerator depends on stealing some of that cold to be cool, so here's my top 3 guesses...
Cause 1 The defrost heater is defective, so frost accumulates on the evaporator coils, and the coils will become plugged with frost causing the airflow through the coils to be restricted, and the refrigerator not to cool. Check the evaporator coils to determine if they are frosted over. If the evaporator coils are frosted over, test each component of the defrost system after clearing any restrictions to the cooling fan and defrosting with a hair dryer.
Cause 2
The evaporator fan motor draws cold air over the evaporator coils and circulates it throughout the freezer. If the evaporator fan is not working, the freezer or refrigerator will not cool adequately. To determine if the evaporator fan motor is defective, try turning the fan blade by hand. If the fan blade does not turn freely, bench test it for function and use a multimeter to test the motor windings for continuity. If the windings do not have continuity, (very odd) replace the evaporator fan motor.
Cause 3
Damper Control Assembly The air damper control opens and closes to let the proper amount of cold air into the refrigerator compartment. If the damper does not open properly, it won't let enough cold air into the refrigerator. Check the damper control to determine if it is broken or stuck closed or that an inadvertent bump has shut it off to the refrigerator compartment