if hdd fails, you should unplug and stop use it

if you still hope to recover user data from it, unplug and seek help from professionals.

if hard drive disk/head are demaged, each power-on might cause more damage to user data.

it's very expensive to recover data from failed hdd, expect to pay more than the value of your computer, while there's no guarantee of any success at all.

again, UNIX101: backup.

if you still want to try it yourself, boot ubuntu livecd from cd-rom, ubuntu can do hard drive (and other hardware) diagnosis, fix possible fs error(if software error only), and recover your files then.