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 do the following:
Remove the charger from the Kindle and hold the power button or switch in the OFF position for about 20 seconds.
Release the button or switch, then connect the charger and try to turn on the Kindle.
If this doesn't work, please repeat the steps and this time hold the power button or switch in the off position for a full 40 seconds. This should reset the Kindle to factory defaults. You will know that it successfully reset to factory defaults, if it reset the Kindle back to the way it was when you took it out of the box. Did it do that? If not, please try it again, being sure to hold the power button for a FULL 40 seconds. If it did require you to set it up again, then the factory reset was successful.
If you do have to reset it to factory defaults, you don't have to be concerned about losing your books. The books that you have purchased from your Kindle will continue to be available at the Amazon website under My CONTENT if you should wish to download them again at some point in the future.
Please try resetting it using the instructions that I've provided, and then try once more to register it. 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! Without a positive rating I am not credited for my answer. If this doesn't solve the problem be sure to let me know so we can continue troubleshooting.