I have just re-installed twice Ubuntu because of this problem, I'm only able to see the login screen when I use recovery mode and select File System Check. In other case I just hear the start sound but I can't see anything on my screen, it still works the terminal because I can press Ctrl+Alt+F1 and reboot the system, but I can't see that on the screen, unless as I said using the recovery mode options. I'm sure it happened after the update because I had been rebooting after any install or copying a file from a USB flash drive and I wasn't seeing any problem before I updated.
I also remember that there was a package that was just downloaded 70% and it automatically changed to another package and I'm not sure it downloaded and installed correctly the package.
Before re-installing, I was not sure about the problem and I even tried configurating gdm and didn't work.