There could be several reasons why your website is down. This could be due to a change on your web host's backend, or an outdated script or plugin that crashed your site, or else it was hacked. For starters, if you have Google Search Console set up, take a look there and see if they detect any malware on your site. Also check your Google Analytics (or whatever you use) to note exactly when the traffic to your site has dropped off; this can help you in your troubleshooting later.
You should access your website through your web host, since your front end login page is probably down. Then go to your website's backend through your hosting company settings. Chances are, if your site is a WordPress or Drupal site, the error is due to outdated versions of either, or to a plugin that's outdated. (This is assuming you're running on a CMS.) After you update your core installation, your theme, and your plugins, the issue may be fixed.
If you're not running on a popular CMS, or if the above doesn't solve the problem, take a look at your installation in your web host's settings. Make sure everything is as you last remember it. Then give your hosting company a call. I have frequently used the hosting company's tech support, and they can do everything from fixing the database setup to informing you if there are infected files somewhere that you have to remove before they can get your site up and running again.