The defrost cycle has quit working and instead of the frost on your evaporator melting off as it should in a defrost cycle isn't anymore being an Amana I would check the termination thermastat first. You have to rtemove the back cover and thaw it out. The T-stat will be clipped to the suction line round and silver in color it's easy to tell if they are bad most the time it's exploded or the top is popped up On an Aman that is generally the problem. You can manually advance it into a defrost cycle by locating the timer and turning it clockwise if by manually advancing it and the heater comes on that tells you the heater and stat are good and the timer is bad cause its not advancing anymore