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This usually happens if the firmware is glitched. First we can can soft-reset the Kindle and see if it works or not. Press and hold down (or slide) the Power button for 20 seconds and this will reset the Kindle. Then check if you are able to unlock it.
If that didn't work, then delete the book and then redownload it. Don't attempt to redownload the book without deleting it first.
Please let me know if that didn't work, so we can continue troubleshooting.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.
Tap and hold down the book you want to delete and select Remove from Device from the resulting menu.
You don't have to repurchase it. Since you already purchased, you will be to download it as many times you want from the archived section when you delete it.
If you don't see the book, please do this. Press and hold down (or slide) the Power button for 20 seconds and this will reset the Kindle. Now navigate to your Archived Items, and will see that book there. Download it please.