sounds like a clogged defrost drain. do you have ice on the floor of the freezer? the drain is likely clogged with food debris, though some get clogged with a putty that some manufacturers use to wrap around a thin capillary tube, and this putty over time can come loose and fall into the drain, or it could simply be clogged with ice. Either of the first two will have to be removed, by clearing the drain tubing, though if you try to do it by sticking a wire down the tube, you could further damage the drain system, in which case I would recommend you hire an expert refrigerator repair service. If its just clogged with ice, and unfortunately there is no way to determine this until after its cleared, but if ice is causing the issue, you will further have to install a drain heater, to prevent this ice build up from happening again.