I've found Google to be extremely forgiving of downtime, particular if the pages are either entirely unavailable, or return a 50x HTTP status (e.g. 503 Service Unavailable).
My expectation is that the worst you may see is a minor blip in rankings, with everything returning to normal. To be honest, I've had pages retuning errors for much longer than a day which were not affected in rankings.
I think Google's thinking is that taking any severe action for a (likely temporarily) unavailable page would be counter-productive, and I would agree with that.
Of course, you'll have missed some crawling which may have a slight impact, and if there are any differences between the old and new, you may have other problems.
Not a problem - 20 hours is nothing to worry about.
20 hours is not a problem. Actually, I have a website that is expired for more than 2 months now but still appear on the SERP.
20 hours should not be a real problem.
I had some issues with my hosting company last year due to which my site was unavailable for nearly 2 days. The rankings were unaffected once things were back to normal.
If Google only looks at your site every few days, you could be offline for a couple of days and they would never know.
Once you go over a week, things start to rapidly slide, and take longer to come back.
Several years ago I was offline for 4 days. Long story.
Rankings didn't budge an inch.
As a counter example, a part of our large site was down for 36 hours and a few days later while looking for causes for lower traffic for several days I noticed that about 1 million of our pages were no longer in the index. [Not suggesting the two are connected but it's all I have to go on, as I don't look at the indexed count often.] Two weeks later most of them were back in the count and traffic levels have returned to normal.
I have had small, personal sites offline for days without a loss in rankings.
I'll add to anallawalla's counter example. We have seen several short-term traffic drops after outages of just a few hours. (Traffic was back to normal after a few days.) It's a huge site and Google comes by all day long. So your actual mileage may vary on many factors, but it DOES seem possible, if not likely, to experience a traffic drop after an extended downtime.
I know of a site that was continously offline for about two months because of a domain ownership dispute. When it returned, it quickly recovered essentially the same Google rankings that it had previously. However, this was an old and large authoritative site with thousands of high quality backlinks.
My site was down much longer than that due to server trouble for like a week by the time I discovered it and reported it to the webhost company. It was getting an Internal server error. It appears I lost most of my ranking and traffic with Google, Yahoo and other search engines. I trust I will gain it all back soon.
My domain has been online since 2000 and it is somewhat of an authority site.
[edited by: Webdetective at 7:59 pm (utc) on Mar. 28, 2009]