If my site goes down for a day, does that affect my rankings?

Matt's answer:

Today’s question comes from Sally, who asks, “I got a ‘Googlebot can’t access your site’ message in Webmaster Tools from my host being down for a day. Does it affect my rankings when this happens?” Well, if it was just for a day, you should be in pretty good shape. If your host is down for two weeks, then there’s a better indicator that the website is actually down, and we don’t want to send users to a website that’s actually down. But we do try to compensate for websites that are transiently or sporadically down and make a few allowances. We try to come back 24 hours later, or something like that. So if it was only just a short period of down time, I wouldn’t really worry about that. The other thing to think about is, you might show up in Google’s Webmaster forum and just look around a little bit. We did have one day a week or two ago where Googlebot itself was having trouble fetching pages. And so, everybody all showed up at once, and they were like, I’m getting this message. I’m getting this message. Even I got that message. And so as long as everybody else is getting it, then it’s clearly something on our side to be fixed, and not something on your side. But we send that as a heads up, because Googlebot usually has pretty good reliability on fetching sites. And so if for some reason it looks like the site was not reachable, that might be something to investigate. But as long as it’s just for a short period of time, like 24 hours, I wouldn’t really stress about that very much.

by Matt Cutts - Google's Head of Search Quality Team

Original video: