If you hold the volume up button at the starting point of boot it will go to a static gear/lightning bolt screen. No progress bar. You can let it sit there for a couple of hours while the battery fills up, and then reboot the device and let it boot into the spinning gears screen.
That is if you can alter bootflow in the middle of the update process. I'm pretty sure you can, but can't verify right now.
Hope you can get it sorted.
IT'S STARTING BACK UP!!! *crosses fingers*