If it's an automatic defrost type, the defrost elements in the back may be burned out (freeser compartment). They are usually under a rather easy to remove panel in the back. On US models they look like small light bulbs about an inch in diameter and perhaps 3 to 6 inches long, depending on the manufacturer and the size of the compartment. Also check the drain line that goes from the bottom of the compartment to the drain tray under the fridge for blockage. Again... this is only for automatic defrost models.