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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 20-30 seconds and this will reset the Kindle.
If that didn't work, then connect the Kindle to your PC with the USB cable and then hold down Power button for 1 full minute and this will force reboot it.
If it still not working, then let me know the model of your Kindle and we can remove the battery and reset it.
Please let me know if that didn't work, so we can continue troubleshooting.
Also reset it while plugged into power. Let me know how it goes.