It's usually one of three things, the defrost timer, defrost thermostat or the element.
Some models have a defrost control (control board)
First for a temp fix unplug the unit, remove the back panel in the freezer use a hair drier to thaw ALL the ice out. Look for your defrost heater see if it looks discolored or if you can see a visible break. Also look for the Def thermostat, a little round thing that has a popup on top and two wires going to it, it the top is popped or it looks chared that's probably your problem. Put it back together and plug it in. It will cool for upto 2 weeks if your lucky(time to get parts in!)
I'll look at the breakdown to see part numbers.
Hope that helps