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Is this model a frost free model or an manual defrost model?
How old is this freezer?
Please post the model number so that we can see what you are looking at and give specifics.
This model (at least the diagram that is shown for it) is not a frost free model. If it were a problem with the frost free system then repair of this would be reccommend and manageable.
You can check to be sure that the fan in the back is turning. If so, replacement of the fan would also be feasible.
However, if the compressor is warm to hot and the fan is running the repair is likey major and would involve either a new compressor and/or replacing the coolant (which from 15 years ago may not be available to you or a servicer).
Freezer is running but not freezing at all. This is most commonly a system problem, compressor, system leak, restriction. Usually big $$$.
A new freezer will be much more energy efficient and in several years will save you enough money to pay for itself (in the money you would have spent on electricity just running this one)!
We see this when a customer moves an older freezer or refrigerator (even if it's across the room to another wall). I'm sorry to have to tell you this. However, a repair servicer in your house would tell you the same.