To power off your device, hold the Lock-Button for one second.

In the popup you tap on “Power off”.

If you decide that you don’t want to, you can just tap anywhere else on the screen.

After it is powered off, you can power it on again by holding the Lock-Button until the Google Logo appears on the screen.

It may take a few minutes until your device is powered on again.

Once it is on, you can just unlock it as usual.