Sounds like a temperature sensor fault. These are located in the freezer evaporator and not an easy item to replace. If you look through the grill vent at the top of the freezer section and can see a solid mass of ice it means that the defrost process is not taking place. The fan is unable to draw air across the evaporator and up into the fridge section. Turn the appliance off for 48hours to allow the ice to defrost. The fridge will then start to cool. If ice continues to build you will need a repair technician.
James
March 2006