I had this issue on my Kindle 6, after not using it for around two months, I had to charge it.
So how to fix this issue?
There is an easy fix, you will have to restart the Kindle. In order to do that, keep the “Start” button pressed for around 3-4 minutes (the “Start” button is the biggest one from the bottom of your device, not the home square button under the screen).
You will notice the loading screen with the tree that you saw when you started your Kindle for the first time (when you bought it).
This works like computers, they have to get restarted in order to work.
Leave in the comments if you still have issues with your Kindle.