One last suggestion -
1.Press and hold the Volume down button and Power buttons at the same time until you feel a vibration (about 10-15 seconds).
2.When you feel the vibration, immediately press and hold the Volume down button until you see a large exclamation mark.
3.Once the exclamation mark appears, press the following four buttons in this order: Volume up button, Volume down button, Power button, Volume down button. Your phone should now reset and restart itself. (It might take a while for the reset to finish.)