I'm having the same problem. I would hold the power button and the n button at the same time and that turns it completely off. Then log back in and hope you can download the app or another suggestion...
I have a nook, but have never tried downloading directly from Amazon. I do have a collection of kindle formatted ebooks which I have put on my nook. The nook will not read Amazon's special AZW format...
The way I successfully do this is to connect the Nook to my computer. Go to your B&N Library, find the book you want to download. Click on "download" which you will see clearly to the right of...