This could be cause by one of the three component of the defrost circuit.
In the defrost circuit there is a defrost timer located at the back of the fridge and the defrost bi metal and defrost heater located behind the rear wall were the ice builds up.
You can see if it is the timer by manually advancing the timer in to the defrost cycle.
To do this locate the timer at the back of your fridge.
Make sure the compressor is running and turn the button on the defrost timer until the fridge shut off.
If the fridge defrost the timer is faulty.
The defrost cycle lasts about 20 minutes.
If the fridge does not come back on you will have to turn the button on the defrost timer until the compressor start up.
If the fridge does not defrost then you have a faulty defrost heater or defrost thermostat.
There is no picture of the heater but here is the part number that you would have to order if it is faulty. The part number is ps34557.
Here is the defrost timer for your fridge. 
Here is the defrost thermostat for your fridge.  | 
