Hi.

First check if the evaporator fan is running.

Then open the freezer door and check if the air is going through the air damper into the refrigerator (up at the top).
If the air blows with the door open and doesn't with door closed - there is the defrost system problem.
Open the rear panel in the freezer and check if the evaporator coils are blocked with snow.

There are 3 things in the defrost system: defrost timer, defrost thermostat and defrost heater.
Locate in the refrigerator top right, rear the defrost timer's shaft.
Use the flat screw driver to slowly turn it CW until the fridge stops.
In 5 minutes check if the defrost heater is on.
If it is - replace the timer Part number: 68233-2