have a ocean frost free fridge freezer?

The fridge works well, the freezer has now stopped freezing, but cold air is still being pumped in.
first it was part freezing, now not freezing at all
Its 3 years old, the first problem i have had with it.
i have noticed a big build up of ice in the air vents at base or freezer underneath a flap, so have turned of the unit to see if this is causing the problem, havent got a hair dryer though!