You have been given some good advice. It sounds to me that your defrost element has failed. It is controlled by the defrost timer which is much easier to get to. You should be able to hear the fan in the freezer when you open the door. If you hear it, I doubt that it is not working properly. If you buy a new seal or need to re work the old one, toss it in the clothes dryer for a short time to get it warm and pliable assuming it has developed a bad "set".
Find the model number and brand of your unit and go to this site:
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It is an excellent source of diagrams, diagnosis, and even parts, though I buy my parts from a local source.