You will need to restore the phone:
1. Make sure iTunes is version 10.7 or later
2. Power phone OFF and disconnect it from the USB cord connected to the computer
3. Press down the home button (circle button on bottome of phone) and then connect it to USB cord.... (do not release the home button until you see the connect to iTunes logo appear on phone...the Apple logo may come up, also, still do not release until the iTunes logo)
4. Release home button when you see the iTunes logo and in iTunes on your computer it will alert you "iTunes has detected iPhone in recovery mode you must restore, etc"
5. Say okay and select the restore button