This is what Amazon recommends for a frozen Kindle:-
Unplug Kindle 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 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 by sliding and releasing the power button.
If Kindle is still not working, you can perform a hard reset by sliding and holding the power button for 15 seconds.
If Kindle is still unresponsive, try charging Kindle before trying to restart the device once again.
Make sure you have the latest software version available installed on your Kindle.
It seems you've done most of that, so if you still can't get it to work, I would give Kindle Customer Services
a call and see what they recommend.