Effective power text message
You can try hard rebooting the device. Hold the power button and the home button for 10 seconds or longer. If you phone does not have certain buttons ie force touch home button/ iphone X please use this article to figure out how to hard reboot your phone.

If you cannot break out of the boot loop and you cannot hard restart your phone, take it in to an apple specialist and if it is an issue with the Operating system they will certainly help you fix it.