Possibly the hard drive has failed and you need to install a new one.
To find out for sure if it's failed, test it with the drive maker's diagnostic software for DOS which loads from a bootable CD. The links are here: http://www.tacktech.com/display.cfm?ttid=287
make the CD from the ISO CD-image by using IMGBurn:http://www.filehippo.com/download_imgburn/
Then boot your faulty PC from that CD.