macOS sierra freezes after startup

My MacBook Pro (Retina, 15-inch, Late 2013) running Sierra version 10.12.2, hung on a grey screen when restarting. Nothing I could do at home, including suggestions given here, could get it to restart properly. At the Genius Bar at my local Apple Store, diagnostics were run, showing no issues with the hardware. The tech then attempted a shut down and reboot, resulting in the same grey screen. She said it was a known issue with Sierra and that some code needed to be entered to tell the computer not to hang up--or something like that. She took the computer in the back, did her magic, and declared the problem fixed. The computer did then restart correctly. Anyone reading this may need to make an appointment at the Genius Bar. This issue will be handled at no charge even if your computer is out of warranty.

shut down

disconnect all external devices

reboot into safe mode

Try safe mode if your Mac doesn't finish starting up - Apple Support

then reboot normally, devices still disconnected.

If the problem persists you may post an etreceheck report for further analysis

www.etrecheck.com

Have you tried to Reset the System Management Controller (SMC) on your Mac - Apple Support