iPhone may be low on power. Connect iPhone to your computer or to its power adapter to charge. See Charging the Battery.
Press and hold the Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, and then press and hold the Home button until the application you were using quits.
If that doesn’t work, turn iPhone off, then turn it on again. Press and hold the Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, and then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears.
If that doesn’t work, reset iPhone. Press and hold both the Sleep/Wake button and the Home
button for at least ten seconds, until the Apple logo appears.