Hello, Thank you for using this service, I will be helping you with your problem today.
this is an easy one, open the freezer and remove all your food and the shelves, then remove the back panel number 10A on this diagram . you will see ice buildup under the coils, take boiling water and pour it over the ice until it all melts and this drains freely again, you can order a drain strap defroster for this to prevent this from happening again. heres the link to order it. http://www.repairclinic.com/PartDetail/Drain-Strap/819043/726493 thanks and best wishes, Bryan