Ringo has two buttons that do the same job, and either one will do it.
One is the power button, just below the screen.
The other is a small button on the right-hand side of the main board, next to the white "ON" marking. Both are shown in the picture below.
If the phone is off, press and hold either button for a few seconds, until the screen lights up and Ringo starts up.
If the phone is already on, press and hold either button until a menu appears offering to power off or restart. Select the option you want and press the A button.
If nothing happens on a long press, a flat battery is the most likely cause. Charge Ringo over the USB cable for an hour and try again.