I would suggest that the fridge evaporater is probably frozen blocking the air flow.
Check the fridge side defrost heater (common failure), also if the evaporater cover has ice on the front of it (behind the water bottle if it has one) or feels heavy when you take it off to see the evaporater then it will also need replacing as the polysterene will be saturated with water and no longer insulating.
Also check if service kit DA81-01997A has been fitted (I can send you the info if you give me an email address) – this is all about a known defrost issue that may be the cause of your problem.
Jem