Hi Karen,
The problem is most likley the auto defrost in the fridge has failed.
This may be just a sensor failure or the defrost heater.
Unfortunatley the heater is sold as a complete package with the cooling coil and sensors.
This means the fridge needs degassing, pipe work reconnecting using specialist Lokring fittings, as a flammable gas is used as the coolant.
The cooling coil is not that dear at around £70.00+vat
But you need an Engineer qualified to use R600a refrigerant, to carry out the repair.
randall refrigeration, September 2009