What has happened to my lovely fridge/freezer?

Model is nearly 4 years old defrost heaters not coming on causing a build up of ice and therefore making the temperature on both freezer and fridge not work properly, drawers not shutting properly because of ice build up behind them, food stored in freezer also getting ice build up. Is this a common fault in RS20CCSV models ?
Anyone else experienced this problem on their side by side fridge freezer SamsungRS20CCSV ?