If your freezer is like mine......I had this happen a few times in a row. What I found was the freezer fans were being blocked with too many food items. It wasn't breathing properly, so ice builds up and blocks the drainage hole in the back of the freezer. This usually happens around the holidays when there is more food then usual, so keep items clear of that area of the freezer. At lease a couple inches from it.

Anyway, once you remove the freezers bottom pan (you don't even have to defrost for this if you don't want to), there is a metal strip along the back of the freezer. To the right of the middle in the metal strip is a small hole that is the drain tube. Usually, if you find ice build up under the pan, there is also usually ice build up on the metal part too and the hole is being covered and frozen over. This causes the condensation that would typically go into the pan under the fridge, to go into the fridge instead. It goes through the opening in the top back of the fridge. Sometimes the condensation pools under the veg bins, or goes onto the shelves. When you remove the bottom pan in the freezer, remove all the ice and expose the metal strip. Take a baster and apply hot water over the hole. Use a dish towel to soak up the water as you will be doing this quite a few (30-50) times until you see water in the catch pan under the fridge. once you see water going through, then add a few more squirts of hot water for good measure. This worked for me. Hopefully it works for you too.
Good luck