In a similar vein, I just had a similar problem, in that for some reason one of my accounts no longer appeared on the selection window on startup. It was the original account, with admin privileges, because I enabled the built-in admin somewhat later. When I checked in User Accounts, the option to use it as a regular account was marked, and when I tried to change it back to an admin account, the button to apply that change would gray out. I don't really understand that, because it has always worked as a backup in that fashion in the past. I just like to have a back door of my own...just in case. I suppose that I can use elevated privileges if needed, but then I will have to get accustomed to that.