[mattsm]

I am assuming that you get the gray screen that says loading on top?
Sometimes it gets stuck at that point, has happen to me, while it is in that state, just force bootloader again, power + camera+ reset hole.
It will force back into bootloader and try to load again.
It should go from loading on that screen to checking and then ask for hitting the power button.

Make sure the .nbh on sd card is on root of card and the only .nbh file on card.
Also make sure the sd card has a format of FAT32.

Good luck, you should be able to get out of this pretty easy.