From a business perspective:

– 1. Think Ahead and Anticipate
– 2. Say Something
– 3. Know the Law
– 4. Remember, You’re Not Alone
What to Do When Your Website Gets Hacked

From a technical perspective:

1. Check That It Has Actually Gone Down
2. Figure Out What Has Gone Down
3. How Bad Is It?
4. Check Your Web Server Software
5. Logging Into Your Server
6. Has It Run Out Of Space?
7. Has It Run Out Of Memory?
8. Has Something Crashed?
What To Do When Your Website Goes Down

And remember, don’t just stand there, reboot something 🙂

From a fun perspective:

Bonus: What Flickr did when their website went down:

AARRON WALTER: Designing for emotion
In July 2006, a storage failure struck Flickr, the popular photo sharing service. Though photos were safe and no data was lost, thousands of enthusiastic users were inconvenienced as
their favorite photo site took a temporary nap (roughly three hours). Tensions ran high as engineers worked to bring the site back online. Inquiries from concerned users poured in.
During the crisis, the Flickr team had a stroke of genius.
[..] They posted a message that explained the outage, asked users to print a page, and do something creative with it to win a free, one-year Flickr Pro account: