Try following things
1) Restart Nook.
Power Off: Press and hold power button for 20 seconds.
Power ON: Press and hold power button for 4 seconds.
2) If the above doesn't work
Go to WiFi Page on Nook and 'forget' the connection.
Then restart Nook using same steps as in 1) above.
Then turn off router for 2-3 minutes.
Then restart your router.
Wait a few minutes and then go to WiFi page and connect to it.
3) If you have access to your Router Settings Page (search online using Google or Bing for your router settings page ip address, it'll be something like 192.168.1.1) then go there and check what the settings are.
a) It should be set to not use dual bands. It shoudl be using a or b or g (n doesn't work fine with Nook sometimes).
b) Some other setting might be off.
c) If you have WPA then try switching to WPA2 or WEP.