At the moment I would look inside the fresh food section and behind the controls there there you will find the defrost timer. On most you need not take the control panel apart but look for a hole with a plastic plug covering the hole. The part is=>
68233-2 Defrost Timer 8 Hour You may also manually defrost your refrigerator by unplugging the power and removal of frozen good to a ice chest. Either use a hair drier once removal of the rear panel in back of freezer or place a bucket of hot water inside and close the door and wait. Sometime beaware that a lot of water will drain to the defrost condensate pan near compressor and may overflow if not careful. If the timer seems to work then we need to test the defrost heater and defrost thermostat=>
MSD2432GRW Freezer Compartment Components
61006116 Heater and Thermostat Assembly
Instructions are on the lower part of that page on how to test with an OHM meter.
Turn the timer clockwise slowly until the compressor and fans stop. That should manually place the refrigerator into defrost. After a minute or so open the freezer door and listen for crackling or sizzle noise. That will be the sound of defrost. Wait about 30 minutes until timer comes out of defrost itself and if not turn the timer again until compressor and fans start again. If this works out like this then replace the timer. If not then we need to go inside the=>
Freezer Compartment Components
Part position 12 is the defrost heater and defrost thermostat. Position 13 is the clip that holds heater in place.
68233-2 Defrost Timer
Should you have questions I will be here to answer, Thanks,
Sea Breeze