If the ice build up is only on the back panel of the freezer then you have a defrost problem. Most likely a defrost thermostat or control board/timer. If frost build up is around the walls of the freezer then warm air is getting into the freezer. Most likely cause of this would be gaps in or torn door gasket or door alignment. In some cases the door was left cracked open. If gasket and door alignment is ok remove the frost and see if it comes back.
Usually answered in minutes!