When we try to restart or switch off our phone through the power button and our phone stops responding. Then it will automatically go through a reboot and this is called a hard reboot. Your phone should respond within 20 seconds I guess after pressing and holding the power button. If not then it goes through a hard reboot. You can also hard reboot your android phone if you want. For that, you need to press and hold the power button plus the volume down button. (simultaneously.) In some devices instead of volume down, you have to press and hold the volume up button along the power button.