I have just had the strangest experience with my Mac Pro. I could not get it to log me in. It would start and get to the login page showing me and guest and then would either shut down or allow me to enter my password and then shut down. I tried all of the traditional multi-key methods for fixing Mac login problems without success. I read something about one of these combinations logging into Guest mode and so I thought I would try guest mode directly. I was able to open guest mode without the machine shutting down. I restarted the Mac again and was able to login to my account.
I have no idea if this was some random event or there was some actual usefulness in this approach. All I can say is that it worked and I thought I should pass the technique on should others have difficulty getting their Macs to start. I will delete this post should this be a temporary fix.