Have you ever been using an app and it’s become completely frozen? You can actually apply a force quit on it by pressing the Home button for at least six seconds.
If that doesn't work, turn iPhone off and turn it on again. Press and hold the "Sleep/Wake" button for a few seconds until a red slider appears, and then slide the slider. Then press and hold the "Sleep/Wake" button until the Apple logo appears.
If the entire iPhone locks up - it can happen - press and hold both the "Home" button and the "Sleep/Wake" switch for eight seconds. You’ll see the screen go black, and then the Apple logo appears as the iPhone reboots.