Any CD/DVD disk in the tray? USB thumb drive connected? If yes, take them out, if no, try to reseat the hard drive. If that does not work, go into the BIOS and check if the hard drive is seen, if yes, go into the Boot Order menu in the BIOS and make sure the hard drive is listed there and not excluded. If all of that did not help, you either have a failed hard drive, or drive controller on the motherboard.