What is this problem please

ADMIRAL
ATF1910DR

Worked perfect all the way up until it didn’t. Freezer and fridge still cool but not enough. Compressor and fan still work, no ice blocking the drain hole but this at the top is a solid chunk of ice and it’s never done that before. I’ve manually defrosted it with a blow dryer several times over the years but that was actual frost and covered the whole thing. If I had money I’d buy a new fridge as this one’s already performed above and beyond the call of duty but I don’t so any advice would be much appreciated. Thank you
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I would suspect the defrost system either the defrost element ( Whirlpool WP61002045 Refrigerator Defrost Heater | Genuine OEM Part ) or the defrost thermostat ( Whirlpool WP61005254 Refrigerator Defrost Thermostat | Genuine OEM Part )