If your phone becomes unresponsive, you can reset it by pressing and holding the Volume up key and the Power key simultaneously for ten seconds.

If you're looking for something more extensive than a hard reset (Volume up + Power key), you can erase all data on your phone and bring it back to out-of-the-box condition. To erase all data and reset your phone:

1. Drag the status bar down with two fingers and touch  Settings > Backup & reset.
2. Touch Factory data reset.

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