Here’s a quick fix for when your Kindle is running slow or freezes entirely:

• Press (or slide) the power button and HOLD for 20 seconds.
• After 20 seconds, release the power button.  (On the Kindle Fire, press the power button again.)
• The Kindle start up screen will appear shortly.

Here are more troubleshooting tips,  with pictures, including a nifty timer to count down the seconds to hold down your power button. 🙂