hi,
could you goto the BIOS and check if the hdd is visible? if yes, then i want you to run system in safemode with command prompt and then run chkdsk with the /f/r commands. If there are no errors, i want you to put in the disc and follow the onscreen instructions for repairing. If that fails too, last resort is to format. Now remember that if after formatting, you encounter the same issue, i would suggest that if the hdd is in terms of waranty, then you need to follow the vendor for RMA and get it replaced, if not, you might have to buy a brand new hdd.