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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.
Do you have access to a computer there ?
Did you check the Archived or cloud on your Kindle ? If you don't see there, then you can resend them to your Kindle using your computer
On your computer, goto Amazon's Manage Content and Devices site i.e go to this website --> http://amazon.com/myk
Log in to your Amazon account. You can find all your purchased books there. Just send or download them to your Kindle (deliver the book to your Kindle using the 'Actions...' menu to the right of the title).
If you have any difficulty, please let me know and I will walk you through it.
Also open Books and check under Cloud or Archived items. Do you see your purchased books there ?
If not, then on your Kindle, go to Settings > device option and de-register your Kindle. Then re-register with the same Amazon account and you will be able to download your purchased books.
Hi there I'm just checking. How is it going ?