The iPhone battery might need to be recharged. For information about charging iPhone, see Charging the Battery.
Disconnect other USB devices from your computer and connect iPhone to a different USB 2.0 port on your computer (not on your keyboard).
Turn iPhone off and back on again. Press and hold the Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears. Tap Unlock and enter your SIM’s PIN if the SIM is locked.
IMPORTANT: If you enter the PIN incorrectly three times, you may need to enter a Personal Unlocking Key (PUK) to enable your SIM card again. Refer to the SIM card documentation or contact your carrier.
Restart your computer and reconnect iPhone to your computer.
Download and install (or reinstall) the latest version of iTunes from www.apple.com/itunes.