Hold down the Power button for several seconds until the 'Slide to power off' option appears. Slide your finger across the red section to turn off your iPad. Once the screen goes completely blank, turn the iPad back on by holding the power button for a couple of seconds.
If holding down the power button does not display the power off screen:
Force your iPad to restart by holding the Power button and home button together until the screen goes blank. Release the buttons then switch your iPad back on by pressing the Power button for a couple of seconds.