Hi.
It sounds like the disk has developed bad sectors. This should be a fairly straight forward recovery for a pro. If the data is important please do not try to access the disk any more as you could cause even more damage such as more bad sectors, firmware corruption and even head damage. This will inflate the recovery costs. You will need a specialist company to deal with this problem.
If you would like us to take a look at it please feel free to pm me, I can give you special HDD Guru discount
www.datatrack-labs.co.uk