What “the site is down” usually means

We get the message. First checks: does the domain resolve, is TLS valid, does the server answer, does the page itself error. Most of the time the site is up and the reporter is on a hotel network, a DNS cache, or a bookmark to http on a host that now forces https badly.

Certificate expiry is the embarrassing one. Calendars exist. We use them. If you host with someone else, put the date in yours too.

If the host is actually gone, the domain should still be yours. Pointing it at a static holding page takes minutes if DNS is in an account you control. That is why the upkeep plan includes the registrar, not only the box.