Samsung Galaxy s3 won't boot
Press and hold VOLUME UP and Home(center) button
Press Power until the phone vibrate
Wait until you see the Android logo then release all the buttons.
Select Wipe data/Factory reset with VOLUME DOWN, press Power (right hand-side)
Select YES -- delete all user data with VOLUME DOWN and then press Power
After format, press Power again to reboot phone. (select reboot system now)