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Your Kindle has probably lost connection with Amazon account. Go to Settings > device > registration and then de-register your Kindle. After de-registering, register your Kindle again and this will re-establish the connection with Amazon and this will allow you to redownload the books.
If that didn't work, then 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 account. You can find all your 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).
Please let me know if that didn't work, so we can continue troubleshooting.