In the bottom of the freezer compartment, at the rear, is a drain hole which leads to a tube. This is probably clogged. Usually, all it needs is to be warmed with a blow dryer (set on low, you don't want to get it hot, just warm enough to melt the ice).
Then, use a thin bottle brush and pour a little water down it to clean out any debris.
If I recall correctly, you'll need to empty the freezer and remove the shelves and coil cover to see the drain hole.
Good Luck