Have you tried going to the HD wireless settings and "forgetting" the connection and reconnecting?
You may need to turn the Nook completely off (not just put it to sleep.) That's what worked when I first bought my HD+.
What BFCoughlin and Petesnook said.
In the rare case that doesn't work - Let us know your router settings and whether it is dual band setting or single band and which band. Which protocol (a, b, n, or g). And other settings.
Nook requires 2.5 KHz and a b or g. N won't work. Dual Band won't work. Rest we need to know your router settings.