Open the case and check all cables
then
The first thing you need to do is test the hard drive with bootable media et al floppy/cdr. You should be able to download diagnostic software from the Hard Drive manufacturer's web site. Alternatively you could test it in another pc to see if it is functional. If the drive is bad that would cause the error
BIOS
Does the drive show up on post?
boot drive order set right?
If all of the above check OK, it is time to reinstall windows, or repair. I would opt for a clean install.