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 won't power on or is unresponsive.
With the device powered on, press and hold the Power button until the "Phone options" prompt appears then release.
Touch and hold Power off until "Reboot to safe mode" appears then release.
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 the device to disable Safe Mode, reinstall then test each app, one at a time, to identify which is causing the problem.