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This usually happens if the firmware/software is glitched. First we can soft-reset the tablet and see if it works or not. Press and hold down the Power button for full 20 seconds and this will reset the Kindle.
If that didn’t work then connect the Kindle to your PC with the USB and press and hold down the Power button for 40 seconds continuously to reset it.
Please let me know if that didn't work, so we can continue troubleshooting