Can you access the BIOS (press F2 as soon as you turn the computer on)
If so check through for boot device settings and make sure the hard drive is set to be the first boot device.
If still no joy, have a look inside, make sure both ends of the cable connecting the hard drive to the motherboard is fully seated and the power lead is in, if still no joy then if you happen to have more than one stick of ram try it with just one at a time in case one of those has gone faulty....