Ok what you will need to do is remove the two screws that hold the lower panel in place. Take the lower panel out and you will need to melt the frost off the coil. Use a steamer
if you have one, if not hot water will work with a towel under the evaporator to catch most of the water. You can use a hair dryer but that will take awhile. When you get the panel off make sure the whole coil is frosted not just part of it. If part of it is frosted and the rest is bare aluminum that is a very bad sign. If it all is frosted then you are having a defrost issue.
The most common thing that goes wrong with the defrost system on that unit is the adaptive defrost control...That is located in the refrigerator section above and to the back right of the controls. You need to look towards the back and there are two locking tabs. Pull down on the trim panel until it clears and then pull the panel towards you....you have to flex it down pretty far.
Once that is off you will see 2....1/4" screws and 2... large head phillips screws...mounting the controls to the ceiling.....there is also 1.... 1/4" screw that is all the way in back that has to be removed. and two small screws towards the back right that are close together that hold the defrost control in place...so you will be taking out 7 screws all together.
When you get them out the panel will come down...You will be able to reach to the far back right to the small square electronic board. There is one modular plug that goes to it....unplug it. If you want to test the heater circuit. Get a small paper clip and stick it in the modular plugs YELLOW AND WHITE wire to the plug(UNPLUG THE UNIT FIRST).....If the evaporator is still frozen the heater should come after plugging it back in. If you are not sure just get a insulate pliers and pull the paperclip out slowly...if you see a spark the heater circuit is working.
Then just replace that control board as it is bad. There is a 99% chance the board is bad and the heater circuit is good. They all went bad on that unit. Here is a link to the control
PART #13 on the diagram so you can see where it is
Now on that uni