I was upgrading my computer from Ubuntu 12.04 LTS to Ubuntu 14.04 LTS,but I turned off my computer during the upgrading, I fixed it so now it's completely back to 12.04. However, when I try to upgrade again, it says:
Check if you are using third party repositories. If so disable them, since they are a common source of problems. Furthermore run the following command in a Terminal: apt-get install -f`
So now what do I do to upgrade it?