Open the freezer, remove the lower 3 racks, then remove the lower rear cover on the back. Under the evaporator coils is a drain trough which is frozen up. Use very hot water poured onto the trought to dislodge the chunk of ice there, then follow up with more hot water to open the drain hole until water drains out on its own into the condensate pan underneath. Follw up with 2 teaspoons of bleach to kill any residual algae.
Use a butter knife or putty knife to scrape the sheet of ice from the freezer floor.
Turn the temp control to where you have 0 degrees in the freezer and 38 in the FF section. If you crank it down too cold, freeze-ups will occur.