With the assumption that your fridge is a residential unit look at the following:
- Open the freezer door, listen for the freezer fan running. Some fridges turn fan off when door is open so you may need to press the light switch. Fan needs to circulate the air
- Next look at the freezer compartment rear panel, check for a frost build up all over the panel. If frost is covering the panel it's a defrost problem. If you find thick frost in just a corner of the panel then it is a sealed system problem.
- If your freezer isn't getting down to zero degrees then you will find the fresh food section isn't maintaining 38 to 40 degrees as it should.
Most likely it's a defrost problem. Frost build up prevents the fan circulating the air. Behind the freezer compartment rear panel is a defrost heater and defrost termination thermostat. There is also a defrost control. Not knowing what model number your fridge is it could be a separate defrost control located behind the fresh food controls or it could be integrated into an electronic control, probably located behind a panel at the rear of the fridge.
If you don't find frost build up then check to see if you can feel air flowing under the front of the fridge (condenser fan). It can be a bit difficult to feel. A light match flame should help with that.
Also if the condenser fan isn't running the area around the door opening that the door seals mate with will tend to be hot (yoder loop). Another sign the condenser fan isn't running. Could have something blocking the blade turning, like a mouse.