If you look at the lower rear panel inside the freezer, you will find a block of ice at the bottom. Your defrost drain is under that ice and needs to be cleared out. Remove as much ice as you can, then remove the lower panel so you can get to the drain. Use hot water poured into the drain at the bottom to melt the ice along with fishing some thing down the drain. I use 1/4" water line that I push into the drain until it gets to the pan under the unit. Dry everything off and you should be back in shape.

Hey Ken, you may want to run something down the drain line just be sure nothing is stuck. Usually you don't find anything, but sometimes you may find things like butcher paper or even peas stuck in there. I use 1/4" water line and push it through until it comes out into the drain pan, that way I know nothing will cause it to happen again to soon.