The defrost system has failed and is not de-icing the coil or, the door is not sealing tight or too much time open allowing excess humidity to enter the chamber, thereby overcoming the capabilities of the defrost system. The system has a defrost timer someplace, check to see if it is running. If not, you can probably advance it into a defrost cycle with a screwdriver. (turn the shaft slowly until a click is heard and the system stops) Allow 20 or 30 minutes for the coils to defrost, and restart with the screwdriver if it has not took off on it's own. If the timer is found not running, replace it.