I've saved all of my epub e-books from my Nook onto my laptop. As long as I have something that can use the epub format will I still be able to access them? Could they still take them away or block access to them if I have copies that aren't on my Nook?
Yes, they could "take them away" if B&N should go out of business and stop supporting the proprietary DRM they layer on top of epub. In other words, your Nook epubs are only readable on a Nook (or B&N-provided applications) that can be unlocked with your B&N account.
If B&N goes away (which isn't unthinkable, at this point) the ability to read your books could go away.