Does the end of the book say something like [End of Sample]? If so, then it thinks you downloaded a sample. If not, then it may be a corrupt eBook.
Try this: go to your online B&N account and to My Library. Delete the book from there. Then go through the "repurchase" and "redownload" process and see if that fixes it.
If the book you downloaded stopped before the conclusion of the book, you were probably reading a "free sample". Check to see if there is a banner on the ebook indicating "sample".
If it is a sample, sometimes it's easier to delete the sample from your "My B&N" page ". Click on the link "Visit your Ebooks Library" then delete the sample. You'll then be able to purchase the complete book.
Note: You should also be able to buy the book directly from within your sample book. There is a green button at the top of the page that says "Buy Now" . Tap the green button (you may have to be at the end of the sample to tap the button), then tap the green price button, then tap confirm. Easy - peasie.