Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
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 40 full seconds and this will reset the Kindle. Then check if you are able to unlock it.
If that didn't work, then connect it to your computer. While connected, press and hold down the Power button for 40 seconds and this will reset it.
Please let me know if that didn't work, so we can continue troubleshooting.