One (or more) of the components in the defrost circuit has failed. Left unchecked, the frost will choke off the cool air supply to the fridge & freezer and temperatures in both will rise.
There are 4 main components to the defrost circuit. They are:
1) Defrost timer,
Some timers are modular and have knobs to manually advance.
2) Defrost terminator, Some defrost terminators have clips to hold them in place.
3) Defrost heater Defrost heaters come in all shapes and sizes.
and 4) Wiring harness. Any of the the first 3 items is a likely source of the problem. The wiring harness is seldom a problem.
The defrost timer is an assembly that consists of a small clock motor and contact switches. The defrost is activated for up to about 20 minutes; 2 or 3 times per day. If the clock motor has failed you will be stuck in which ever mode it failed in - your's would be stuck in the compressor mode. If the contacts have failed, it will not be able to send power from the compressor to the defrost circuit. It will have 4 or more wires - and often is a plug in wiring harness type device. These will only connect one way - unless you force it.
The defrost terminator is a simple temperature sensor. It is often clipped directly to the refrigerant coil - but may be anywhere in the refrigerant coil compartment (which is most often found behind the back wall of the freezer itself). It operates like a simple switch. It is closed when the coil or compartment temperature is below about 50 degrees F (normal cooling or compressor mode). When the defrost timer switches from compressor mode to defrost mode, power is passed through the terminator to the heater. When the temperature rises above this point (the defrost heater has been energized and melted the frost) it opens, and stops power from reaching the heater, preventing it from continuing to heat. This device has 2 wires and there is no "polarity" or "wrong way" to wire it.
The last is the defrost heater. Like the defrost terminator, it too is located in the refrigerant coil compartment, and almost always directly below it. It is a fairly low wattage, two wire heater that also has no polarity and can't be wired wrong.
You'll need some simple test equipment (volt meter and ohm meter) to test these components successfully.
I hope this helps and good luck!