Lets start by going inside your fresh food compartment and see if you can find a round hole most times covered by a plastic plug to hide opening. Remove plug and place a flathead screwdriver in opening and slowly turn the defrost timer clockwiseuntil you hear compressor and fans stop. This is called forcing manual defrost. After a few moments you should hear some sizzle or crackle noise coming from freezer compartment. That will be the defrost heater warming up the unmelted frost. Click Here=> ER2CHMXPL00 Liner Parts
See item location 4? That is the defrost timer While the freezer has been set manually in defrost the defrost timer should be trying to come out of defrost itself after about 25 minutes. If it does not come out of defrost simply turn clockwise until the compressor restarts. If you need to do this replace the defrost timer.
482493 Defrost Timer Kit
If doing the above does not help you will need to manually defrost the freezer with a hair drier. Unplug refrigerator and remove the rear panel in back of your freezer. This is where you will see the defrost heater. Click Here=> ER2CHMXPL00 Unit Parts
2315530 Defrost Heater Assembly
On that same page instructions are in a video on how to test the defrost heater. At top of this solutionI mentione the Liner Parts item number 25 is the=> 4387503 Defrost Thermostat
Most likely the defrost timer is the part that is needing replaced. If not then the other two parts listed here are very possible reasons for the defrost failure.
ER2CHMXPL00 Repair Parts List
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User Manuals can be found here=> Whirlpool
I will be here should you have any questions, Thanks Sea Breeze