If the evaporator coils behind the back panel of the freezer are icing up because of auto defrost failure that will stop the circulation of cold air
check defrost timer, defrost heater, defrost thermostat. You may have an electronic control instead of a timer.Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com

If the heater and thermostat are ok it’ll be the control
Your evaporator coils frost up in normal use and every eight hours or so the entire unit shuts down and the defrost heater comes on to melt the frost. This cycle last about 20 minutes. The melted frost drips into a drain pan and through a drain tube to the drain tray under the freezer/refrigerator where it's evaporated by the condenser fan.