Perhaps you have exceeded the access timespan of your book. Please check your bookshelves. Drag the shelves from right to left to make sure your book is not off-screen. Click on the title MY BOOKS on top of the bookshelf and check whether you have chosen to hide your book. View your activation history by tapping Settings 
My Profile. A list of your book activations appears at the bottom of the page. Look for any expired books.