This from Amazon's website:
Unplug Kindle Keyboard from power adapter or computer.
Slide and release the power button. The LED light that surrounds the power button will display green for two seconds.
Shut down Kindle Keyboard by sliding and holding the power button for five seconds. The LED light will blink three times. Wait for the screen to go blank, then release the power button.
Turn on the Kindle Keyboard by sliding and releasing the power button.
If Kindle Keyboard is still not working, you can perform a hard reset by sliding and holding the power button for 20 seconds.
If Kindle Keyboard is still unresponsive, try charging Kindle Keyboard before trying to restart the device once again.
Make sure you have the latest software version available installed on your Kindle Keyboard. The latest version is available here:
www.amazon.co.uk/kindlesoftwareupdates