Tough one. Well if your only server goes down, then you don't have a choice but to bring it back online, then communicate clearly as to what happened, what you're going to do to take care of it and prevent it from happening.

Run your business based on the communication and you'll see good response from your customers. They're usually forgiving in most cases, unless they're the sub-five dollar type that want 24/7 phone support and 1000 GB of disk space...