This happened to me for the first time today. I wanted to upgrade and put in the root PW, but on clicking enter the log-in screen just disappeared. After I had tried twice more, I thought I was entering the wrong PW (my wife's computer). So I entered another PW, but on that attempt it just told me that this was the wrong PW. So I did what I always do in case of emergency: I rebooted. After that, no problem. Go figure.... But thanks for all the possible solutions.