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This usually happens if the firmware/software is glitched. First we can soft-reset the Kindle and see if it works or not. Press and hold down (or slide) the Power button for full 40 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 60 seconds continuously to reset it. Then check the book.
Please let me know if that didn't work, so we can continue troubleshooting
Also make sure screen rotation is not locked. Click the Settings icon (the gear icon at the top right) or swipe down from top of the screen. This opens the settings toolbar. The far left icon will either say Lock or Unlock. If it says Unlock - click it. That will unlock rotation again.