Hello! My name is***** am a computer professional with over 15 years of experience with both Mac and Windows computers, phones, tablets, and printers. This problem and many others are often the result of a "glitch" in the software. Luckily, it is easily fixed. Please give me just a moment to research and try to figur out exactly which Nook you have so I can provide the proper instructions. If you see any type of serial number of model number on the back of the Nook, please let me know, as that would help tremendously.
Listed below is generic information for troubleshooting and resetting your Nook. This will work on most models.
Your Nook is likely rarely shut down, as just pressing the power the button puts the Nook into sleep mode. However, if you press and hold the power button for 5-10 seconds, your Nook should will pop up a box asking if you want to turn it completely off. If you tap 'Yes', it will fully turn off. Wait a minute or two and then try to turn it back on.
If it still won't turn on, we will need to do a factory reset. A factory reset will de-register your device, removing all digital books and files, including side-loaded content. The titles you have purchased will remain in your account.
NOTE: It is recommended that all side-loaded content you wish to save is backed up to an SD card or computer before proceeding.
To Hard Reset your Nook, complete the following:
Please give this a try and let me know how it works for you. If this solves the problem, would you mind taking a moment to rate my service? You can give your rating by selecting the stars located on the top right of the question page. I'd really appreciate it! If this doesn't solve the problem be sure to let me know so we can continue troubleshooting. I'll need to ask a few questions in order to determine the exact model of your Nook.