Ubuntu 12.04 upgrade failed, now unuseble system

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http://askubuntu.com – I performed an upgrade from 11.10 to 12.04, after a few hours the system restarted itself. I saw a login prompt on the display, but the system does not respond to my keyboard or mouse input. Tried to restart the system multiple times, still the same. Any recommendation on how to recover from this and still keep the program and datas I had on the system? Anyway to revert it back to pre-upgrade state? (HowTos)