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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.
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.