Charge up your battery and it will boot just fine. The problem is the Multiboot preventing the original kernel from booting. This is why it can't charge the battery. Find someone with a Nokia 5800 or N900 and swap the battery. Get a spare battery or something would help alot.

flashing mode will not work when low battery. Try leave the phone plug into power to see if the original kernel is loaded and charge. if you see orange light then its charging else get a spare battery. Else get ride of Multiboot or nitDroid.