Google Nexus S hard reset
1. Press volume up then power until robot
2. Select recovery with volume down and press power
3. then you will see robot + exclamation you must hold volume up again + power
4. Choose factory data reset and wait until phone is wiped
5. Choose reboot now once wiping is done
Press VOLUME UP + POWER when you see this exclamation mark with droid
With the Nexus S turned off Press VOLUME UP + POWER on the phone.
In the Recovery menu select (with Volume DOWN) RECOVERY, then press POWER.
When you see the exclamation mark and the Android, you should hold down the power button and then press the volume up key, while still holding the power button. Don't press them both at the same time.
Nexus S will reboot, After Google logo Android appeares. If it won't reboot after a few minutes, just take out battery, then reinsert it.