When was the last time the tablet was turned off (not just put into sleep mode)?
To turn off the tablet, hold the power button for two or three seconds, then tap "shut down" when asked if you want to shut down your Kindle.
After the device has completely turned off, press the power button to restart.
Please reply with more information if this did not fix your problem!
See the following page for more information:
Quick Fix Slow or Frozen Screen