Press the power button and hold it down for several seconds. The power button is usually on the right side of the device.
After a few seconds, a menu should appear with the Power Off option. The latest Android version might offer other options, including Restart, which is a better option.
Even when the Android operating system can't display the power down menu, you can perform a hard reboot, also known as a hard restart; this is different from a reset or manufacturer's reset. Not every Android device is programmed to do a hard reboot the same way.
Many devices reboot when you hold down the power button. However, it may take 10 to 20 seconds before the system reboots.
If the operating system doesn't respond, try holding down both the power and volume up buttons for up to 20 seconds. After that, the screen will go black, signaling that the device has powered down.