Virtual Server: Network cable unplugged
Man, this is one thing that had me going for a while. If you have instances setup to run under a specific user account (i.e. to auto-start on power up), you need to ensure that the user accounts have the proper permissions for all of the virtual files that is uses. This is nothing new and the web interface takes care of the virtual machine file permissions for you, but it doesn't manage the permissions for the virtual network adapters. If your virtual network adapter files are in a different folder, as they should be, this can be a problem as the virtual machine may not be able to read the files.
While tightening up security on my servers, I removed the Users group from the root folder that holds all of the virtual files. The next time that I rebooted one of the virtual machines, all of the network connections on that virtual machine reported "Network cable unplugged". To fix the issue, I gave each of the user accounts that the virtual instances are running under, Read & execute permission to the folder that contains the virtual network adapter files and rebooted the virtual machines. You will also want to ensure that the Network Service account has Modify permissions in order for the web interface to function properly as well.