Back wall on the inside of my fridge icing up?

Hi,i have a Candy CPCA291KXV frost free fridge freezer and when opening it this morning everything in the fridge compartment was frozen?Freezer section is fine incidentally.The back wall of the fridge is covered in ice?Any idea what i can try to get it back to normal?It’s only 3 years old and has done this once before a year ago,can’t remember what i done then though to get it working normally again.