Unsuccessful upgrade from 11.10 to 12.4 LTS leads to black screen with cursor blinking

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http://askubuntu.com – During the upgrade from 11.10 to 12.4, I got stuck on a screen with background image, and nothing else. I restarted the computer, and after GRUB menu, screen gets black with cursor blinking. I've tried getting into text mode, by removing quiet splash, and by adding text, but it leads to the same black screen. So, I cant get to both text or graphical mode. I'm stuck on GRUB menu. What can I do to get it working again? Thanks! (HowTos)