When package upgrades are interrupted in midway, normally doing
sudo dpkg --configure -a && sudo apt-get -f install && sudo apt-get upgrade will save the situation, but when is a full release upgrade (jumping from a version to another) the steps becomes more tricky.
What's the best solution for this cases? Reinstall. If you are not capable to reinstall, you may want to continue the upgrade using
sudo update-manager-text or
sudo apt-get dist-upgrade (the last one you shall modify your
sources.list from oneiric to precise, see my command at the end), but that I won't guarantee will work. Other option is
sudo do-releases-upgrade, but you must make sure that the process is not interrupted this time around.
sudo sed s/oneiric/precise/g /etc/apt/sources.list
sudo apt-get update
sudo apt-get dist-upgrade