Most public WiFi hotspots are configured with NO link security and, at best, just a firewall block that requires on to accept a terms-of-service agreement on a web page before being able to connect through the firewall.
Home systems, except for those installed by the truly clueless, should have one or more of: link encryption (WEP, WPA, WPA2), MAC address filtering, non-broadcast SSID (the access point does not transmit its name -- you have to spell it to make the first connect).
Note: there has been some indications that the Nook does not work well with WPA security (WPA2 is okay) nor on mixed b/g/n WiFi (few are using slow b, but n really wants to be the only mode on the channel -- save n for 5GHz routers, use g for 2.4GHz which the Nook can find).
Tell your Nook to forget your connection (router) then search for available connections (I assume an SSID is broadcast), and enter the security key for that router (not your B&N account password). Actually, check your router set up first to confirm 2.4GHz g with WPA2 or WEP and what the key is.
I am having the same problem. I started about 2 weeks ago. Hope someone knows how to fix this.