Alternatively, it could be a problem where your hard drive isn't connected properly, either due to IDE (or SATA) failure, or motherboard connection failure, or power supply failure. If this happens, I first of all fiddle around with the connections on the inside to your Hard Drives, and if this doesn't fix it, try reinstalling the OS as in the previous instructions. If the reinstalling software (such as windows) does not detect the Hard Drive, then it is a detection issue probably due to the aforementioned reasons, or less likely, a BIOS issue (but this shouldn't occur because I am assuming this computer has been working in the past for a while).
-Stephen
