fridge freezer ?

Hi fridge freezer stopped working from one day to the next.it seems to get warm inside but not getting cold.It has a verry big build up opf ice in the freezer compartment.When i turn the thermostat i cant hear it click,there is a buzzing noise coming from the back compressor but the fridge is still not working ,please help ,can it be the thermostat at the back of the fridgr attached to the compressor that needs changing? many thanks zm