My site is down: what do you check first?

First check whether it is really your site. Half of these cases are local.

Is it down for everybody?

Test from your phone on mobile data, off your own network. If it works there, the problem is your connection or your local DNS, not the server.

What is the exact message?

A 500 error is an application problem. A connection error is a server or network problem. A certificate warning is an SSL problem. Three different causes.

Did something change?

An update, a deployment, a DNS change or an unpaid invoice. Almost every unplanned outage follows an action.

Does email still work?

If mail works and the site does not, the server is running. If both are down, the problem is more fundamental.

What you write down

The time, the exact message and what you did just before. That is precisely what support will ask, and noting it now saves a whole round trip.