The source – searchenginejournal.com
Google employee John Mueller, during a recent video meeting for webmasters, answered a question about the negative impact on positions and SEO for a site that has been inaccessible for more than a week.
Mueller said it's impossible to minimize the effects of such prolonged downtime. If a site is unavailable for longer than a few days, Google starts de-indexing its pages.
During the day, it's best to use server response code 503. But after a couple of days, Google starts to regard this code as permanent and remove pages from the index. When they become available again, Google will try to re-scan them. But they may not immediately return to prominence, there may be large fluctuations in the ranking until the situation is normalized.
So if the shutdown lasts longer than a couple of days, you should expect at least fluctuations in the ranking. It is better to avoid it.