How to Restore Your iPhone
Sometimes, you must restore your iPhone to make it behave. To restore your iPhone, connect it to your computer as though you were about to sync. But when the iPhone appears in the iTunes source list, click the Restore button on the Summary tab. This will erase all your data and media and reset all your settings as well.
Because all your data and media still exist on your computer (except photos you’ve taken, and contacts, calendar events, and On-the-Go playlists you’ve created or modified since your last sync), you shouldn’t lose anything by restoring.
Your next sync will take longer than usual, and you may have to reset settings you’ve changed since you got your iPhone. But other than those inconveniences, restoring shouldn’t cause you any additional trouble.