Blank screen after upgrade from 11.10 to 12.04!

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http://www.unix.com – Hello, I got blank screen when reboot after I tried to upgrade Ubuntu (64 bit server version) from 11.10 to 12.04 at update manager. The normal login GUI is missing, but mouse cursor is fine. Then I switched to terminal (tty1 ~ tty6), which looked fine and showed it is Ubuntu 12.04 ! (Forgot how I got it, but I did see it is 12.04 and I was reminded Code: 12.10 is available! The upgrade seems completed, at least partially, but the logon GUI is missing! I had thought the upgrade is broken, and have tried many different ways to fix it: Code: 1) sudo apt-get update && sudo a (HowTos)