Hold power button and volume button down together for 15 to 20 seconds and then release them together. Then hold power button normally like you would go turn it on. If it doesn't start__ try the steps again. This rebooted my wife's Fire hdx in the past. She has used the Fire every day for over 4 years and it is still a great tablet. She no longer uses our computers but uses the Fire for everything including printing to a wireless printer ( she downloads tons of knitting patterns and prints them from the Fire)