Turn off applications that aren't being used. Use extended standby mode when screen is inactive to minimize battery consumption. Lower screen brightness level.

Make sure you have set your device to use the correct network type. If your subscription plan does not support the LTE (4G) standard, you must select a network type other than LTE (4G). If necessary, contact your operator for more information about your subscription and the network types supported.

All you have to do is hold the Volume Down button and then the Power key and hold these two buttons down simultaneously for about 15-20 seconds. After holding them down for about 20 seconds if the phone didn't power cycle then release the keys and try and turn the phone on like you normally would by briefly pressing the power button.