There are several components and systems involved in the cooling process of a refrigerator. I would reccomend you look at the back wall of the freezer compartment for frost or ice, indicating a defrost problem,if heavilly blocked with ice this will restrict air flow and unit will seem to not cool.
You may also need to check the defrost thermostat(located on the evaporator coils behind the back wall of the freezer compartment)or temperature control and defrost timer circuits located in the control panel assembly in the fresh food compartment. If the contacts in your temperature control stick open the control will not reset and the compressor will not run. If the contacts on the defrost timer are not resetting after a defrost cycle, the compressor circuit is not completed and the compressor will not run. If the thermostat does not reset, the control interprets that as still in the defrost cycle and will not allow the compressor to run and cool the unit likewise.
The least likely situation would be that the compressor relay or the compressor itself has failed , at which I would reccomend a servicer be contacted and finish the diagnosis and repairs