Am not going to do research and give you a answer here, I have faced a same problem in the past.
Please hold the power button for a longer period. It might turn off, but then make sure you have your charging cable beside you.
Step 2 : Hard start or turn on the device by holding the power button for a longer period, device might vibrate and proceed to its booting screen, make sure , your wifi is turned on.
This might fix the problem. Good luck !