Follow these steps:
1st List: Hard Reset
1. Hold the home and sleep/wake button until the screen turns off.
2. Let your iPhone reboot. If it doesn't start to reboot after 20 seconds then hold the sleep/wake button for about 5 seconds or until the Apple logo appears.
2nd List: DFU Mode
1. Hold the home and power button for exactly 10 seconds.
2. After 10 seconds, release the power button and continue to hold the home button for 10 seconds.
3. Release the home button after 10 seconds.
4. Attempt to power up your iPhone. It should power up but if it doesn't then connect to iTunes. A message should pop up saying and iPhone has been detected in recovery mode. You must restore it...
4. Click restore and follow any on screen instructions.
3rd List: Deactivate and hard restore
1. Open iTunes on your computer (Mac or PC).
2. Power off your iPhone by following the 1st step of the 1st list of instructions.
3. Hold the home button and connect the iPhone to your computer with iTunes open and holding the hoe button.
4. Soon, a screen should pop up saying connect to iTunes with the iTunes logo and a USB cord.
Hope this helps!
David