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.
With the device powered off, press and hold the Power button until the moto logo appears then release.
Approximately 2-3 seconds after the moto logo appears on-screen, press and hold the Volume down button. If pressed too soon, it will not boot into safe mode.
Continue to hold the Volume down button until Safe mode appears in the lower-left of the screen then release. This may take up to 45 seconds.