Hi! My name is ***** ***** I'm happy to help!
Connect the charger and press and hold the power button down for 20 seconds. This will force a reboot and hopefully clears any errors. If you've already done this and it still freezes up, reset the device to factory. All of your files are safe on Amazon's server so long as you remember your Amazon email address and password.
To reset to factory perform the following: CLICK HERE for a video
Once it is back up, login and your books will be available for download. Please reply if you get stuck or need more help.