Have you cleaned the condensor, the condensor needs to be be cleaned every 3 - 6 months.  if you have cleaned the condensor and your fridge still isn't getting cold.  Check to see if the fans are working.  The internal fan is inside the fridge and the external fan is where the grill is at the bottom of the fridge.  One side will suck in and the otherside will blow out.  If the condesor is clean and the fans are all working it will most likely be a refrigerant issue.