It could be a variety of problems: Does it sound like the compressor pump is running or not? Are the cooling fins blocked with dust & debris to the point that they can't radiate the heat from them? Maybe it sprung a leak and the freon escaped out of the system?
Here is a link that explains how refrigeration works: http://en.wikipedia.org/wiki/Refrigerator
Whether you want to get it fixed or just buy a new one depends on how old the freezer is and how much a repair shop will charge VS the price of a new freezer at a local store.
Hope this helps,
BILL