I can't get your model number to pull up. But generally, start by locating the defrost control, under the control panel in the frig part. see if you can advance the timer until the unit shuts off, if after a full turn on the little knob (which normally requires a screw driver) the unit doesn't turn off then the defrost timer is the problem.
If it does shut off listen for the sound of ice popping and maybe a sizzle sound. Remove the rear panel in the freezer. if you don't hear that locate the defrost thermostat (bimetal) should be around the evaporator coils, it's a little round part about the size of a quater and about an inch thick with 2 wires attached. Test that with an ohm meter for continuity while it's really cold. If it has continuity then use a hair dryer to defrost the coils and inspect the heat element.
Let us know what you find.
Nat

Need to try to get it to shut down using the defrost timer, not the temp control. It's a defrost problem, which means the timer, defrost thermostat or the defrost element has failed. There should be some screws on the rear wall of the freezer compartment.
Nat

No screws. shouldn't the whole floor just slide forward? I can see a seam between it and the walls, I just can't figure out how to get it out. I took the control panel loose to see if that was holding it, but it looks like it should just slide right over the control panel and out the front.

I got it out! It does just pull out, it was held by the ice.

Now I'm going to try to thaw it out.

Thanks!

Dave

The disk I was talking about turned out to be the freezer temp control which controls an air door between the freezer and the refrigerator. It is functioning fine. I never did find the timer.