My Dell which was 5 years old suddenly karked it and I had the "low graphics" black screen and useless dialogue box. As I believed it was a dead graphics card I went out and bought a brand new machine. I put aside the new machine and tried again in vain to open the Dell. I eventually got to the command line via Ctrl+Alt+F1. I logged into my account from there, and simply started a series of sudo apt-get remove of various softwares that I knew were installed on my PC (software without any great consequence like Google earth, tweak, Skype etc). Lo and behold after a sudo reboot my computer was fine again! So now I have 2 computers. BUT one week after buying the other one and installing 12.04 because I love Ubuntu, the SAME PROBLEM arrived! I once again deleted Google earth, Skype, and did a sudo reboot and everything worked as before. I think there is a bug or something in 12.04 as this problem has never arisen with any other versions of Ubuntu.