The hard drive may still be functional. The operating system has become corrupted.
You could run a fitness test on the hard drive or if you need to recover files NOW, use a live version of Linux, like Puppy, to access the files and copy them to flash or optical media. If the files are not accessible because Puppy can't see the hard drive, then the drive may be damaged.
After that, try performing a restore to factory state.
You have learned about one of many reasons why you should maintain backups of your personal data.