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 can be powered on and is responsive.
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With the device powered off, press and hold the Volume Down button.
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While still holding Volume down, press and hold the Power button for about 5 seconds until the boot mode screen appears.

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Select Safe mode.
Utilize the Volume buttons to scroll through the menu options and the Power button to select.

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With Safe Mode enabled, test device and app functionality.
Related Topic:
Using Safe Mode