Do a hard reset. Hold down the home button and the sleep/wake button at the top for about 15 seconds. The power off message will come on. Continue to hold it. The phone will cut off. Continue to hold them. When the apple logo comes on, release the buttons.
If that doesn't work, do a brand new restore on Itunes. You will have to set up your phone as brand new. Make sure that you have your contacts saved somewhere or you will lose them. If that doesn't work, buy an Iphone 4.