There are a multitude of reasons why a site can go down. These reasons include, but are not limited to: Content changes, server maintenance, internet connectivity problems, and the stability of third party services that your website relies on.
If you've recently made content changes which may have caused the error, you can revert them to see if this restores the site. But in most cases, these sorts of outages are usually out of your hands.
Regardless of how, why and when a site goes down, the right thing to do is to let us know. Give us a call, send us an email, or raise a support ticket and we'll look into it as soon as we can.