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.
With the device powered on, press and hold the Powerbutton (located on the top-left edge) until the "Phone options" menu appears then release.
Touch and hold Power off until the "Reboot to safe mode" prompt appears then release.
From the "Reboot to safe mode" prompt, tap RESTART to confirm.
The restart process may take up to one minute to complete.
Upon restart, Safe mode appears in the lower-left of the unlock/home screen.
With Safe Mode enabled, test device and app functionality. If the previous issue fails to occur, the cause has been isolated to a third-party app and all recently installed or updated apps should be uninstalled.
Power cycle the device to disable Safe Mode then reinstall and test each app one at a time to identify which app is causing the problem.