SAMSUNG RS21DNS FRIDGE NOT COOLING?

The freezer works fine, but the fridge has started to become warm at the top of the cabinet (thermometer shows 12 degrees C), but it is cooler at the bottom. The digital display on the door shows 2 degrees. This happened 2 weeks ago, but it corrected itself after a couple of days, but today it has happened again.
It is not connected to a water supply.