I have a customer with a Server 2012 R2 domain controller that wouldn't boot. It would get to the logon screen and then, after a few minutes, reboot.
I booted into safe mode and, using MSCONFIG, concluded that the Security Accounts Manager service, SamSs, is the issue. I set to run all services (Normal Start) except this one service and the computer boots.
With SamSs disabled many other services won't start.