how can i replace thermostat? BUSH FROM ARGOS?

I come back home today to find the back of the fridge frozen up and freezer not working. Have removed the switch which works fridge and freezer?
Now it doesn't work at all. However the compressor.motor still works when I switch it on. Machine is 3yrs old and out of warranty. Any help would be grateful and seen spares on amazon.
Would it sort it or could it be something else?