What could cause the freezer to work but not the fridge?
Usually there's a fan that moves air from the freezer to the fridge.  It could be that the frost is preventing the fan from spinning and/or blocking the air chase between the two.  Would probably take a week or so to build up to that point.  If the defrosters not working, which, if that's the back of the interior of your freezer, it looks like it isn't, that's definitely what I would start checking.

I'm not fridge guy, but my first step would be to verify voltage to the defroster.