Beko fridge freezer icing up?

My mothers beko fridge freezer gets very iced up at the back of the fridge. I made sure the drain hole wasn't blocked. If I turn the thermostat down, the freezer stuff starts to thaw. it's meant to be self defrost. The freezer part is now icing up, and has stopped the little fan from working, tho' I have tried to get the ice out of it My mother may have left the freezer door open. Is it a thermostat problem?