Short answer

Check, in order: the service is active in the client area; the domain has not expired; the domain points at your hosting; and the site itself is not showing an error. If all four look right, open a Technical Support ticket.

Checks

  1. Service: in the client area, the hosting shows as Active, not Suspended. If it is suspended, see my service has been suspended.
  2. Domain: under Domains, the domain has not expired.
  3. DNS: the domain points at your hosting. After a change this can take a few hours; see checking DNS properly.
  4. The site: an error page with a number (500, 503) usually comes from the site, a plugin or a recent update. Check the error log in the Website Panel, or restore a backup.

When you open a ticket

Include the address of the site, what you see (a screenshot helps), when it last worked and anything that changed. That saves a round of questions.