Download the latest version of iTunes.
Turn the iPhone off by holding down the "Sleep/Wake button" until the phone is completely shut down. Do not have the phone hooked up to the computer.
Launch iTunes.
Hold the "Home" button down and connect the iPhone to the computer. Keep the button pressed until iTunes shows that the phone is in recovery mode. Release the button.
Follow instructions as they appear on the screen. The procedure will use a previously created backup and should restore the phone.
See if that helps.
Source: eHelp