Jason:
Make sure you unplug your computer before opening up the case again. Then, check that the Molex connector from the power supply (the one with four pins and red, black and yellow wires) is firmly connected to the hard drive and not upside down.
Next, check that the IDE cable (the flat "ribbon cable" with 40 or 80 wires) is not inserted upside down and is not loose but firmly connected to both the hard drive and the motherboard. Both the motherboard and the hard drive may indicate which side of the connector is the location of pin number one. Make sure the wire with the red line on the IDE cable is connected to this pin.
Finally, be sure to check that the small jumper is not missing and is connecting the correct two pins on the hard drive (as Merlin suggested).
If that doesn't work, let us know and we will tell you how to check the BIOS setup configuration to make sure that the system is set to boot to the hard drive first.
Regards,
Doc