Just imagine, you go to your website/portal/online store, but it is... gone. Or it is still there, but it looks like some drunk hackers had fun with it. What is the reason for the disappearance or breakdown of the site? It can be anything: a virus, actions of intruders, a physical breakdown of the server, software failures, actions of your developer, or even your own exercises with the site.
You will deal with the reason later. In the meantime, the site needs to be restored. Because while it is down, new customers go to competitors, old ones can't place orders or go to competitors, too. Business has stopped, sales are dwindling.