Safe Mode puts your phone in a diagnostic state (returned to default settings) so you can determine if a third-party app is causing your device to freeze, reset or run slow.
An alternate method is available if the device is unresponsive or won't power on.
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Press and hold the Power button until the Power off prompt appears then release.

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Touch and hold Power off until the Safe mode prompt appears then release.
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To confirm, tap Safe mode.
The process may take up to 30 seconds to complete.
Upon reboot, "Safe mode" appears in the lower-left of the unlock/home screen.
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With Safe Mode enabled, test device and app functionality.
Related Topic:
Using Safe Mode