It is going to be expensive. Drive Savers and Ontrack Data Recovery are both good. If this is worth in the high three figures or low four figures to you, talk to a data recovery firm immediately. Do not attempt to recover the data yourself--anything that you do the storage medium is likely to increase the cost of recovery and decrease the chances of it being effective. The worst possible course of action is to attempt a recovery yourself, fail, and then go to a professional service.
That said, if this is not worth that kind of money to you, there is software available that can help to recover data from corrupted storage media. Just don't start there if the data are truly valuable to you.
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