firstly, check that the thermostat wire is still connected properly to the fridge internal cooling plate . The thermostat wire may have become dislodged and is not in contact with the cooling plate.
If its ok then replace the thermostat. You can fit a universal one for about 20 quid. But ensure that the wire that comes out of the thermostat switch makes full contact with the internal cooling plate - or that part of the inside of the fridge that gets cold first.
neil rogers, October 2005