Waited too long to log-in on boot lock screen, now my Android is constantly restarting. How can I fix this?

I turned on my phone and as the bootup screen was working my attention drifted to another task, by the time I looked back the Sophos boot lock screen had turned the phone off, only to restart it directy to the the home screen (with android log-in).  About 30 seconds later, it shut itself down.

Now my phone cycles through this every 30 seconds or so.  On and off.  Any advice appreciated.