Here's another handy self-help suggestion I found when I googled:
It sounds like you have a plugged defrost drain line. This is where the water goes to in the bottom of your freezer when it goes through the defrost (removal of ice from the freezer coil) cycle. It sounds like that hole is plugged and now the water/ice has now frozen in the vent that lets air circulate down into your refrigerator compartment. What I usually do in this case is to unplug the refrigerator, empty out the freezer compartment. Usually I also end up either pulling off the back panel or the bottom panel (in the freezer compartment) Looking for that defrost drain hole and thawing it out with a heat gun (be careful of melting plastic) or actually I use a hair dryer. I can sometimes do this also by pouring small amounts of HOT tap water onto it and then sopping/absorbing it away with a sponge or dishcloth. I repeat this until I get the water to go down the drain tube. I hope this help everyone.
Boy, me too! I had to do this recently. Fridge was still cold, but I had a pond forming in the bottom, under the crisper drawers. Pulled out the freezer interior and thawed the drain with a blow dryer. At least a hundred dollars cheaper than a repairman. I searched answers.com using "why is there a puddle in the bottom of my refrigerator?"