There are two kinds of freezer, and the same symptom means different things in each.
A manual defrost freezer — a chest freezer, or an older upright — is expected to accumulate frost. Clearing it once or twice a year is maintenance, not repair, and it is worth doing because a layer of ice insulates the walls and pushes running costs up.
A frost-free freezer, which is nearly every fridge-freezer sold in the last two decades, is supposed to clear itself. It runs a heater against the evaporator several times a day, melts the frost, and drains the water away. Visible ice on a frost-free machine means that cycle has stopped happening.
The symptom that usually appears first
Most people notice the fridge compartment getting warm before they notice ice, because on nearly all fridge-freezers the cold is made in the freezer and blown into the fridge by a fan.
When the evaporator ices over, the fan is pushing air through a solid block. Airflow drops, the fridge warms, and the freezer often stays cold enough that nobody suspects it. So a warm fridge with a cold freezer is one of the most reliable indicators of a defrost failure, and it usually develops gradually over one to two weeks.
There is a test that confirms it without opening anything. Switch the appliance off with the doors open for 24 to 48 hours so everything melts, then run it again. If it cools normally for a day or two and then fades in the same way, the ice was the problem and the defrost system is what failed. A compressor or sealed-system fault does not behave that way — it does not get better for two days after a thaw.
What actually failed
The defrost system has three parts that commonly fail, and any one of them stops the cycle.
The defrost heater is an element wrapped around or below the evaporator. When it fails open, nothing melts.
The defrost thermostat or bi-metal cutout tells the control when the coil is cold enough to need defrosting and warm enough to stop. A failed one leaves the heater switched out permanently.
The timer or control board decides when to run the cycle. On older machines this is a mechanical timer; on newer ones it is part of the main board.
From the outside all three look identical, which is why this is usually diagnosed by a technician with a meter rather than by inspection.
Frost that is not a defrost failure
A door left ajar, or one that no longer seals, lets warm humid air in continuously and produces frost faster than any defrost cycle can clear it. Check the seal by closing the door on a strip of paper and pulling — it should drag all the way round. A drawer or a package pushed against the seal will do the same thing.
Frost concentrated around the door rather than on the rear panel points at the seal rather than the system. So does frost that appeared suddenly after a holiday, a power cut, or a delivery that involved the door being propped open.
Very humid rooms and frequently opened doors both increase the load, and a freezer in an unheated garage in winter behaves oddly for a different reason: many models rely on room temperature to trigger the compressor at all.
Why not to leave it
Ice is an insulator. As it thickens, the freezer works harder for less effect, the compressor runs longer, and running costs climb. On a fridge-freezer the fridge compartment eventually stops holding a safe temperature, which is a food safety issue rather than an inconvenience.
Scraping the ice off buys a few weeks and does not address why it formed. It is worth doing once as a diagnostic — because if the fridge cools properly afterwards, that confirms the airflow explanation — but not as a routine.