you have an original hard-drive of some sort right, like a 20 gig or something that came with your 360 that you wanna make work again? This is assuming that you hooked it up to your pc and then suddenly it won't work anymore on the 360 (you didn't take it apart or break it by accident, it just got hooked up and now it doesn't work).
What happened is your motherboard went and added something to the hard-drive that angers the v360.
http://www.xboxhacker.net/index.php?topic=12062.0Read through that and get that program that you need to boot from a cd or something. Follow the instructions and it should be able to fix that problem.