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 powered on
With the device powered on, press and hold the Power button (located on the right-edge) until the Power off prompt appears then release.
Touch and hold Power off until the Reboot to safe mode prompt appears then release.
Tap OK to confirm.
The restart process may take up to 45 seconds 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 device to disable Safe Mode then reinstall and test each app one at a time to identify which app is causing the problem.