Check to be sure you haven’t disabled the Virtualmin Universal repo (look in /etc/apt/sources.list for Virtualmin repos), and make sure it is working. From the command line run apt-get update and check for errors and fix them if there are any. A search of the forum will turn up several conversations about fixing updates not working on Ubuntu. They changed the signature requirements a while back, though I don’t think that effects 14.04, which upgrading to using the new vm/6 repos will fix (and is covered in several forum topics).
You’re also running on an unsupported distro, but the virtualmin-universal repo will continue to work for some time to come (and there is a new universal repo in the /vm/6 subdir).
In short, you need a line like this in your sources somewhere:
deb http://software.virtualmin.com/vm/6/gpl/apt/ virtualmin-universal main
And, you need to import the vm6 key:
wget -qO- http://software.virtualmin.com/lib/RPM-GPG-KEY-virtualmin-6 | sudo apt-key add -
Edit: Also, make a plan to upgrade your OS to at least 16.04 (but preferably 18.04 or 20.04) or migrate to a server running a newer version. 14.04 is not safe to run on servers.