Try doing a power down and a power up -- hold in the power button for ~20 seconds, ignore the "do you want to shut down" message and you'll power all the way off. Then power back up, and see if the book is able to be downloaded.
If that doesn't work, and you have Nook app on your computer, you can download it there. You can copy epub files from the computer to the NC and the NC can read them using the account info stored on it, as long as your computer and your NC are registered with the same email address.
I just brought in the preview on the Nook app, so I get the feeling that the book is available now.
I would try archiving it and then unarchiving it. You may have to do this through your bn.com account for it to work but probably not. For some reason a lot of pre-orders won't download unless they are archived first. I don't know why, but I see it a lot.