I have a Frigidaire Refrigirator - about 15 years old I think. It's one of those two doore refrigirators with the Freezer on the left and the refrigirator on the right

Yesterday I noticed the Freezer was no longer working (and the refrigirator side also seems to be not as cold as usual).

The freezer is still cooling, but its more like a stronger refrigirator - not strong enough to freeze the ice

I noticed on the back of the freezer there is a thin layer of ice

Any ideas whats the problem? or how much it would cost to fix it? would it be worth it to bring repairman or should I start looking for a new fridge?