smeg refrigerator not cooling
It sounds like the defrost cycle is not working. Ice builds up on the evaporator and eventually blocks the air flow to the refer section. Defrost is set to occur usually once or twice a day. Most common failures are the defrost timer or the defrost heater. Sometimes though, it is the defrost termination sensor that is found strapped to the refrigerant return line. If you can locate the timer, check that the timer motor is turning . Some timers can be advanced with a flat blade screwdriver in order to start the defrost cycle. 

The heater will be found embedded in the fins of the evaporator coil. (good luck trying to replace the heater) When the defrost is working you should be able to see some of the water from the melted ice. The water drains from the evaporator into a catch pan located under the unit where it evaporates. Squido