The first thing you'll need to do is pull everything out of the freezer and remove the entire back panel inside the freezer. That includes removing the shelf and the icemaker if you have that option. Look at the silver evaporator coils behind this panel and are they packed with frost, lightly frosted from top to bottom, some frost on top but not on bottom, no frost at all, etc... ? Also, is the fan above the coils running?
If the fan isn't running, you need to replace it.
If the coils are completely packed with frost, you have a defrost problem. You'll need to thaw out all the frost with a hair dryer and use a multimeter to test continuity through the defrost heater. If the heater has no continuity then replace it, otherwise replace the 1" round thermostat on the top of the silver coils which is your defrost thermostat.
lastly, if the coils are only partially frosted or no frost at all, you have a freon leak or an inefficient compressor. You'd need a technician to check the freon pressure on the low side to determine which issue you have but neither are inexpensive options. A freon leak is about $450 to repair and a new compressor is $650.
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