Cold air is produced on the freezer evaporator and blown forward. A small foam damper, or a motorized one, meters that air into the other section. Two failures feel identical from the outside: the fan is not turning, or frost has buried the coil so the fan has nothing to move.
You will not see the fan run with the freezer door open. The door switch cuts it. Listen at the freezer door a minute after you close it. Silence, with the compressor still humming at the bottom of the cabinet, is the clue. A fan that rattles and then stops has a blade hitting ice.
Frost over the coil also means the defrost circuit missed a cycle. A heater, a defrost thermostat, or the control board can each be the failed part. They are not interchangeable, and the frost looks the same until someone opens the evaporator cover. That cover is clipped in place and the foam around it tears if it is pried at random. Leave it for the person with the parts.
A compressor problem looks different. Both compartments drift warm together, and the housing at the back bottom is either dead cold or hot enough that the overload is clicking. Do not replace a start relay because the milk is warm while the ice cream is still firm.
The full symptom list is on the refrigerator page.