remove your storage devices and try to boot it. if it boots fine then you have to find the bad files and remove them from hdd. if it still freezes with no hdd or usb in then your getting RROD

Well sounds like you downloaded the wrong files, and instead got the ones from someone that causes your jtag to brick. Or like slapah0 said you are getting rrod which sounds like it. I had the same thing happen, but i would check to make sure that it is not just the files. You could also try reflashing your nand if you have it or have it reballed. Other than that there is pretty much nothing else you can do

XenonNSM wrote since i have downloaded the online files my jtag freezes at start up , +rep for help

It is the lunch ini on the hdd. you can fix this by not useing the hdd or get a transfer cable and go on xport360 and deleting it.(transfer cables are about 4 bucks on ebay)