Then Start your iTunes and connect your iPhone to your mac/pc.
2) Keep holding the sleep/wake and home button for about 10 seconds. Release the sleep/wake button, but keep holding the home button. you will hear some sounds.
3) If you did it correctly iTunes will tell you that it has detected an iPhone in recovery mode.
4) If you see the iTunes icon and cable on your iPhone, your device is not in DFU mode, but in recovery mode. Your device’s screen will be black when you are in DFU mode. I always have to try this several times so succeed.
5) Connect your device back to your Pc/Mac and press SHIFT+CLICK (pc) or COMMAND+ CLICK on the restore button.