Brett just updated to OSX Mojave and hangs up on the login screen. Leo says clearing the PRAM or parameter ram can sometimes solve issues like this. Also, reboot with CMD R and that will enable you to reinstall the OS. Maybe it wasn't updated properly. If that doesn't work, Leo recommends making an appointment at the Genius Bar in the Apple Store.
Chatroom says to boot into safe mode, press and hold the shift key, and it will install the absolute minimum. If it works OK after that, then there's an app that isn't compatible that's hanging things up.