Ubuntu 12.04 not starting up

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http://askubuntu.com – I have just recently upgraded from Ubuntu 11.10 to 12.04. I am running simultaniously with windows 7. Ubuntu 11.10 used to start up no problem but ubuntu 12.04 just doesn't, it hangs on a black screen. I have tried DVD installs and downloaded upgrade install but both produce the same results. I have even tried Ubuntu 12.10, still no boot. Please help (HowTos)