Try resetting your BIOS, on the motherboard there is usually a jumper near the battery you can move to a position to clear the BIOS to default. Unplug the PC power before doing this. Then reboot and see what happens. You may want to reseat the hard drive cables as well while you're at it.
If that does not help, as Grumpy said, your hard drive died. Hopefully you can find someone to see if any of your files are recoverable.