Hi, my name is Farrell & I will be helping you with your problem today. Thanks for your question, it is appreciated. Everything your frig. does, is controlled by the main control board, located in the backside of the frig. just above and to the right of the compressor housing. What has happened to your frig. is the defrost system has failed. There are 3 main parts that will need to be replaced, in order to get your frig. back in to working order again.
WR50X10028 defrost termination thermo. $13.39 located on top of the evaporator/cooling coils.
WR51X10042 Heater assy. $70.92 located at the bottom of evaporator
WR49X10138 main board $222.99 on the outside of back wall
All of these parts are available at sears parts and servicenters. Or;
These parts are available online at repairclinic.com
I have suggested 2 different locations for you to choose from. Again, it will take all three of these parts to fix your frig. You will need to be sure the frig. is unplugged before servicing. Also, be sure the evap. coild are clean and clear. Let me know how you come out on this. I am here, if you have further questions. Have a good evening, and thanks.