first of all forgive me for my bad english...
this problem could occur if you're tweaking XBMP setting.... and you accidentally changed your EEPROM from NTSC to PAL.
the reason there's no picture because xbox thinks that it's PAL, but the video decoder is actually NTSC.
now we have to make the xbox think it's ntsc again
so download a program called video_select_v2_enigmah-x.rar from xb**s it's located in /XBOX/apps/utilities/video mode switch/ folders.
click here for a faq to go to xb**ns
and make a boot disk from the xiso contain within that file,
afterwards, put that boot disk into your xbox,
wait a few seconds until the programs load and hit button B to make it back to ntsc