I recently faced the problem of not being able to update Ubuntu 12.04 to 14.04 in my DigitalOcean droplet.
Despite these great instructions: https://www.digitalocean.com/community/tutorials/how-to-upgrade-ubuntu-12-04-lts-to-ubuntu-14-04-lts
It did not work because there were missing files.
I had to manually edit the sources list
sudo nano /etc/apt/sources.list
And uncomment the original DEB package sources, and comment the Digital Ocean mirror packages.
Hope it helps!