yeah sounds like your chip isn't booting properly. You're getting error 05 becuase the chip is being by-passed and the box is launching the original mictrosoft bios, hence the error 05 for your unlocked HDD.
I'm not sure why the chip would spanteously stop booting. Pull the box down, double check all the connections and try again. If you still cant launch XOS, try performing a XOS recovering. You'll need to burn and insert a recovery disc (seee the tutorials section), then pull the box apart, flick the switch on the chip into the recovery position (recovery is sliding the tab toward the top of the chip), put the HDD and DVD-ROM back on, power up by eject and see what a happens. Do you get a splash screen with a picture of a Xenium chip? if not what?