| I've had the same problem with an upright freezer. These units typically have automatic defrosters. What that means is, occasionaly a heating element will turn on and melt the frost that builds up around the freezer coils. Problem is, the temperature of the bottom of your freezer compartment is still below freezing so the water freezes again. There is a little hole there that the water is supposed to flow down and out to a pan that is near the floor under your fridge.
My remedy is not very hi-tech. I simply empty the freezer and put the stuff in camp coolers. Then pans of hot water go into the freezer until the ice is melted. Have some sponges and towels at the ready. Be patient. You'll fix it.
Find a piece of wire that you can run down that hole to be sure that the hole is cleared. Partially cleared you can pour hot water down it 'til it's totally cleared.
Then you might consider turning your temperature control down a notch or two. | 
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