It would be nice to a screenshot of the password dialog even if you need to take it with a camera, so we can see what kind of a lock it is. It would also be useful to know the exact model of your computer, and that of the hard disk if you can find it. And also if the drive in question is the system drive or not.
This is important, because if it is really the hard disk that is locked, then there is no simple way of unlocking it. Normally, its firmware will even refuse to reformat the disk without the password. This can happen because someone locked the drive in the BIOS, or because of a hardware glitch.
The best option for you in this case is to get in touch, first with with the Support for your computer manufacturer, then (if that doesn't help) with that of your hard disk. You will need a proof-of-ownership of your computer to have a hope of getting an answer.
As far as software is concerned, the article Unlocking a password protected harddisk claims that the atapwd program can do the job. The links no longer work, so this and other such tools may be found on this Hard Drive Tools page. However, be warned that using these tools is quite risky to your hard-disk and computer and that I do not have any experience with any of them.
A commercial service may be found at Repair Station, and again I have no experience with them.
There is a person that claims unlocking his drive using two hard disks. What he did was set a password in the BIOS on his unlocked disk (meaning lock it), reboot and go into the BIOS to clear the password. The BIOS will ask for the password to be typed in, so enter the password from above, and when it then asks for a new password replace physically the original hard disk with the locked one while the computer is still turned on and then enter an empty password, followed by as many OKs as necessary.
I have no idea what will be the result of the above procedure and take no responsibility.
Best Answer
With the information given so far, the only answer with a high probability to work is to employ a commercial data recovery service. If the data is really valuable, consider doing this. In this case, trying to solve the problem yourself may do more harm than good.
There are many possible causes, drive's internal hardware, firmware, connector,... . Have you had a look at the Related questions on the right? For example, How do I recover data from my presumably dead hard disk.
Try connecting the drive to a computer that is running a different OS. For example, boot Linux from a disc.
You might get more focused help if you stated the exact model names of both drive and computer, and told us possible reasons why the drive failed to work (impact?).