When I ran Software Updater I was given an error message saying
Failed to download repository information, check your internet connection.
This is what it says:
W:The repository 'http://ppa.launchpad.net/upubuntu-com/icons/ubuntu xenial Release' does not have a Release file.
W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
W:See apt-secure(8) manpage for repository creation and user configuration details.
E:Failed to fetch http://ppa.launchpad.net/upubuntu-com/icons/ubuntu/dists/xenial/main/source/Sources 404 Not Found,
E:Some index files failed to download. They have been ignored, or old ones used instead.