You'll need to connect the phone to iTunes then, do the following;

Hold down the sleep/wake button (top button) and the home button (round button) at the same time until your device turns off. Release the sleep/wake button but, continue to hold down the home button. You should see a notification from iTunes indicating it has found a device in recovery mode. Then you simply click on the restore button in iTunes. This should resolve th issue. Hope this is helpful.