Step 1: Press and hold the Sleep/Wake button until the red slider appears.
Step 2: Drag the slider to turn your device completely off.
Step 3: After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.
How to force restart
You should force restart your iPhone, iPad, or iPod touch as a last resort, and only if it's not responding. To force restart your device, press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.