[ubuntu] problem with upgrading 11.04 to 12.04 LTS

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http://ubuntuforums.org – yesterday we were upgrading an 11.04 ubuntu pc to 12.04 and it took so much time..while on the process, i shut the computer down because we have to go home..(the pc that we were upgrading is a school property)..i didn't see any "cancel" button on the screen so i just shut the computer down.. later this morning, when we opened the pc, the background is just black,it has a computer icon in the center of the monitor and and "x" button on the upper right corner.. what should we do?? help!!! (Hardware)