I have a 1.6 XBox which I modded some time ago with a Duo X2 GS (blue PCB) chip.

Today I turned on the XBox, and there was no audio or video at all, but the red light from the mod chip was still flashing as usual.

I tried booting to the Microsoft dash (turn on using eject button), and it worked fine.

I tried the on button again, but there was still no picture or sound.

What should I do? I remember seeing a thread where someone was having a similar issue - he fixed it by switching the chip to it's second bank, and then flashing it with the 1.6 BIOS. I tried booting the XBox with bank 1, but there was no A/V, and the light on the front of the XBox was flashing green and orange.

try going over the lpc and freshen up your solder points

it sounds like your evoxM8(1.6) bios bank has become corrupted.
the other bank does not have a 1.6 compatable BIOS.

in short, you need to flash both banks of the chip with a 1.6 compatable BIOS image, but you can't do it with a 1.6 xbox.

other option...buy another chip.