Nobody wants their site to go down. After all, if your site isn’t working, that means all of your conversions are on hold until your site is back up again. A big concern for SEO, in this case, is what will Google see if your site isn’t working? This topic has been covered previously on Search Engine Land, and John Mueller even tweeted about it recently as well.
Is your site offline for less than a day?
If this is the case, all you want to do is use a 503 status code. This will let Google know that your site is temporarily unavailable, so come back later and see if it’s back up. According to John Mueller, be sure that the server is showing a 503.
Is it offline for over a day?
If your website goes offline for over a day, then yes, there will be a bit of a problem. URLs will be dropped from Google’s index, regardless of if you’re using the 503 status code. In this case, John Mueller says to hack together a solution.
Source – Search Engine Land