My problem began when I have restarted my laptop after I have installed a new GUI, I have Ubuntu 12.04 with ATI mobility HD 5470, but all was fine and working good. On my Ultimate Edition 3.4 I have Google-d how to fix that and someone suggested that :
sudo apt-get remove lightdm then sudo apt-get install lightdm
would solve the problem, but my problem was I only could apply
sudo apt-get remove lightdm, then it wont take the install lightdm order, and I ended up with only a black screen after I boot and choose Ubuntu from my dual OS booting (I have Linux and win7), I cant even switch or see tty. I have worked on my Ubuntu a lot and don't want just reinstall it again and lose all I have built in it before.