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, simultaneously press and hold the Power button and the Volume down button (located on the right edge of the device ) until the Sony screen appears then release.
From the System Recovery screen, select Safe mode.
Utilize the Volume buttons to navigate and the Power button to select.
Allow approximately 1 minute for the device to reboot.
Once rebooted, "Safe mode" appears in the lower-left corner of the screen.