How can I repair my american fridge freezer?

We have a Rangemaster American style side by side fridge/freezer U109100-02. The freezer is working perfectly but the fridge temperature went from 5% to 15%. We turned it off for 48 hours to allow for any defrosting turned it back on it went back to 5% but has now gone to zero so is freezing items in the fridge.Any ideas what could be wrong with it - it is only 3 years old.