First make sure that your site is actually down and that it isn’t your internet connection. Visit other websites to see if they are reachable. Also try visiting your website on other devices or have a friend visit it from their computer. If it is down, one thing you should always do is notify your users that your site is down. You can do it through social media or you can set up a backup webpage that will show a “We’re working on it!” page.
Temporarily direct your domain to that webpage until your actual website is back online. Let them know what is going on and when to expect your website to go back online. It is good courtesy to keep your users in the loop so that they know you care.