So, you turn the computer on, and it beeps and displays what on the screen? Does it give an error code? Do the lights on the disk drives come on, then go off, or are they on solid? If they're on solid from the moment you turn on the computer, then you have a cable plugged in upside down...
Or does it give you an error after reading from the disk for a bit? How many drives do you have connected, and what kind? Is there a drive connected after the twist in the cable?
-Ian