First try a "soft restart" of your Kindle. Begin by pressing the Menu button. If device is unresponsive, skip ahead to "hard reset." No worries - this should not erase all your books!
Scroll down and select "Settings."
Press the Menu button again.
Scroll down and select "Restart." WARNING: Do not select "Reset to Factory Defaults" - this will erase all data from your device!
You should see messaging confirming that your Kindle is restarting.
Finally, you will see the start up screen, indicating you have performed a successful restart. Hopefully this has resolved your issue!
If your device is frozen or unresponsive, you can also try a "hard reboot." Do this only if you are unable to restart from the device.
Press and hold the power switch for 15 seconds. Yes, a full 15 seconds - no fast count! The green light will illuminate, and then flash twice after 6 secs and most screens will go blank. Keep holding!
Your screen will go blank after ~6 seconds - keep holding!
Let go after 20 full seconds. Shortly after you let go, the green light will illuminate again.
You should see the start up screen appear, letting you know you have successfully restarted your device. Hopefully this solves your issue!