One of your defrost components has failed. There is the timer, the thermostat and the heater itself to check. I usually start by advancing the timer to the defrost cycle (when everything is still frosted up) if the heater starts working then I know the timer is not advancing on its own and needs replacing. If not then it's either the heater or thermostat. Unhook the heater and check for continuity (fridge unplugged) if Ok then it's likely the thermostat. The only way to properly test these is when they are still frozen and again check for continuity.