if you did any soldering then check the points very carfully you may have burnt the baord, i did on my first mod, ended up parting it out and selling everything on ebay.
if you didn't solder then you could have messed you the xbox bios some how, i couldn't tell you how to fix that,
but there are some tutorials on this site to help you fix your board if you did burn a point, if not search google.
