do is the relatively small solder point on the backside of the motherboard. By soldering to it, it provides a zero amp short to the ms bios which allows the bios on your matrix chip to be loaded instead. Because d0 is such a small solder point, and because you say it worked yesterday, my guess is the the wire came loose and the reason you can't launch a dash is because there is a conflict with your ms bios trying to boot and your matrix bios trying to boot through the LPC points. But it's just a guess.
Anyway, take your xbox apart. Pull out the motherboard, flip it over (unless you soldered d0 onto the via point on the top of the motherboard, either way it's the wire that *isn't* connect to the LPC port, perhaps it is grey) and make sure the d0 solder point is still secure. If it isn't, I reckon, that's yer problem.
hal.