Fedora 28 has just reached end of life, almost 13 months since it was released. If any of your systems are running Fedora 28 you’ll have to update to a newer version as packages in the Fedora 28 repositories are no longer receiving security, bugfix, or enhancement updates; this leaves your system vulnerable to attackers if you connect to the internet.
Users are now being advised to upgrade to Fedora 29 or to Fedora 30 in order to get updates for software. Upgrading your system shouldn't be too difficult with the clear instructions provided but if you do run into any issues another option is to download a fresh copy of Fedora 30 and perform a clean install. If you do opt for this method, be sure to backup all of your important data as it will be erased.
The Fedora life cycle is quite interesting, rather than setting an end of life date when a new version is released, updates continue for one month after the second subsequent release of the version in question. On May 1, the Fedora Project released Fedora 30 which was the second release since Fedora 28, therefore, four weeks later, Fedora 28 stopped getting updates. This cycle is good in those times when a Fedora release is delayed for some reason or other.