Hi there and welcome
You will need to do a full reset of the Nook, it will wipe all data and return it to new, but this cannot be avoided sorry, it is the whole reason for the lock.
To do the reset
1. Ensure your NOOKcolor is powered off; to do so, depress the power button for 5 seconds. Your LCD screen should turn off
2.With your NOOKcolor powered off, Hold the Home Key (NOOK “n,” logo below the LCD), while simultaneously depressing the Power Button for up to 25 seconds.
3. Your NOOKcolor will begin the start up process, Do NOT let go until a confirmation window appears asking if you would like to, “Factory Reset Your Device?” This will De-Register as well as erase all content and information on your NOOKcolor (Including the 4 Digit Pass-code
4. Depress the Home Key to continue with the Factory Reset
5. Depress the Home Key again to Confirm
6. Your device will now begin the clearing process. Upon completion you will regain access to your NOOKcolor however all account and personal information (email, contacts, pass-code, credit card, etc..) will be deleted from your device. You will then be able to Re-Register your NOOKcolor and start anew
Let me know how you go please