Did you try
sudo apt-get update
sudo apt-get dist-upgrade
sudo do-release-upgrade
This should upgrade your system to 14.04 from 12.04 without any problem
How to upgrade from Ubuntu 12.10 to 14.04 [duplicate]
Did you try
sudo apt-get update
sudo apt-get dist-upgrade
sudo do-release-upgrade
This should upgrade your system to 14.04 from 12.04 without any problem