Depends if the problem is physical or software which is impossible to say without further information. If its physical your best bet is data recovery company.
If problem is only in filesystem and you have access to software such as Disk Warrior, Techtool Pro, etc, you may be able to repair damage or recover data. If it fails you could try recovery software such as Data Rescue which might be able recover your data. These aren't cheap software and if your data is critical I would recommend using data recovery company.