This and many other Issues are often caused by a software "glitch". Luckily this can be easily resolved. Please hold the power button or switch in the off position for 20 seconds. This will force your Kindle to power off, rather than just "sleep" as it usually does. Hopefully that will resolve the issue.
If this doesn't work, we can RESET TO FACTORY DEFAULTS. The MOST important thing to know about doing a Kindle Fire reset to factory defaults is that your tablet will be WIPED CLEAN. This means it will return to its original state when you first bought it (except for any physical wear and tear, that is).
It will no longer be registered to your Amazon account, and all of the content (apps, games, videos, docs, books, etc) that you download, sideloaded, emailed, or added in any way to your Kindle Fire will be gone. Also, all customisations, game data, stats, bookmarks, etc will be erased.
After the reset, you can re-register the Kindle Fire to your Amazon account and download any content that is in your cloud. Any book or game data that uses Amazon's Whispersync feature will also be available.
To do a factory reset please press and hold the power button or switch in the off position for a full 40 seconds. This should reset the Kindle to factory defaults. You will know that it successfully reset to factory defaults, if it reset the Kindle back to the way it was when you took it out of the box. Did it do that? If not, please try it again, being sure to hold the power button for a FULL 40 seconds. If it did require you to set it up again, then the factory reset was successful.