After installing macOS Sierra, it wouldn't boot. Sometimes it would hang, sometimes it would crash. Booting in safe mode and uninstalling AVG antivirus fixed the problem. If you are planning to upgrade, remove AVG before you do. If you are in a boot loop, try safe mode (power on while holding shift) and remove AVG (just move it from Applications to the trash, no special uninstall process).
I have been struggling for 6 hours and nothing.
Did as you suggested and BINGO!
You're welcome. Exact reason I posted this...