I upgraded my Ubuntu 11.04 Oneric to the latest version Ubuntu 12.04 LTS this morning, but after installing all upgrades & cleaning up (Upgradation steps), when it restarted first time to complete installation, it got hang. Then I removed VGA cable of external display attached to my laptop & did a hard reboot then booted successfully and again when I connected other display, it got stuck again.
NOTE: I did upgrade to 12.04 LTS with 2nd display connected & that combo was working well with 11.04 Oneric.