Ice build-up in a fridge can cause it to go warm up. As ice is an excellent insulator, this prevents the fridge from drawing out heat to keep the fridge cold.

Usually, ice build-up is from moisture that enters a fridge which eventually cools and starts forming ice blocks.

The following reasons are usually the cause of ice-build-up:

Doors frequently opened
A frequently opened door or a door left open for long periods can also cause ice build-up.

Damaged door seals
Check if the door seals have any cracks or broken pieces. Damaged door seals allow moisture to enter which speeds up ice build-up. Repair damaged seals with super-glue or replace the perished seals.

Defrost not working
There may be an issue with your defrost timer, defrost sensor or defrost elements.

A qualified technician should test this for you to find the cause.

Faulty thermostat
The fridge thermostat could be faulty and prevents the compressor to turn off. This causes the compressor to continuously run to negative temperatures, therefore the fridge begins to act more like a freezer than a fridge.

Not enough gas (refrigerant)
If there is ice build-up on one side of the fridge, this is a clear sign that the gas level is too low. What happens is that there is not enough pressure for the gas to exit the evaporator, therefore only freezes on one side.

A qualified refrigeration technician will be able to sort this out for you.