If on the iPhone it shows a white usb cable pointing up to the iTunes symbol then the iPhone is in recovery mode. Just plug the iPhone into PC with iTunes open and a prompt should pop up saying that "iTunes has detected an iPhone in recovery mode you must restore". Press ok and follow the prompts to restore the iPhone to factory settings.
You have to download iTunes into your computer. It would take a few minutes to install iTunes into your computer.
Thereafter, iTunes would detect your iPhone in recovery mode. Proceed to recover. Any error message, a pop-up window would direct you to a diagnostic site in Apple. Follow the procedures given and re-do the recovery of your iPhone. Sometimes you need to reboot your computer in order to get a successful recovery.