You may also want to check the various cables. Unplug and plug back in both/all ends, and if you have the CD drive and hard disk on the same cable (assuming the hard drive isn't a SATA, with just a small thin cable to the motherboard), then make sure the hard drive is set to master, and the CD drive to slave - or put them onto a seperate cables as two masters.
As oilrig says, when you do come to boot it up again, check that it's set to boot off the CD drive, then hard disk (though you'll probably change it back to HD first when it's all otherwise working).