if the server is showing up with a version number, like below, you're on the wrong version of minecraft, check the pinned messages in #rules-and-announcements for the right version to be on
if the server is showing up with the text "Pinging..." staying there for a while, like below, simply click the refresh button and it will work properly, minecraft just does this sometimes
if it persists after doing that, check #rules-and-announcements for something saying that the server is down, if there isn't anything, mention in #reports-and-suggestions that the server is down
if the server is showing up with the text "Can't resolve hostname", like below, check #rules-and-announcements for something saying that the server is down, if there isn't anything, mention in #reports-and-suggestions that the redirect is down
if you have a problem that isn't listed here, report as much information about it as you can in #reports-and-suggestions