Frost building up in freezer
A common cause for this problem is failure of the fan unit in the back of the freezer compartment that circulates the air inside the refrigerator/freezer. You would want to determine if this fan is still working properly. 

Note that the fan is typically hidden behind a plastic panel and not directly visible. Exact details regarding how to access and remove the fan will vary according to the make and model of your refrigerator.