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If for some reason you can't reset the phone using the Settings app, then you'll need to reset the phone with a hard reset. This means you'll be using the phone's hardware to reset it, instead of a software app. The power button is on the upper right side of the phone. Press and hold the power button until you see the Power options screen. Touch Power off to turn off the phone. Wait until the phone is completely off. The volume up/down buttons are on the left side of the phone. While pressing and holding the volume up/down buttons, press and hold down the power button. When the Samsung logo appears, stop pressing the power button, while continuing to press and hold the volume buttons. When the Android System Recovery screen appears, stop pressing the volume buttons. Using the volume up or down buttons, highlight the Wipe data/factory reset option, and then press the power button to select it. Press the volume down key to highlight Yes - Delete all user data, and then press the power key to select it. Press the power key again to reboot the phone. The Samsung Galaxy S2 will begin the reset process. Do not turn off the phone while it is resetting.

Summary:
Try resetting with the Settings App first. Turn off the phone. Turn on the phone with the power and volume buttons. Reset the phone.