There was a time when Googlebot would pay visit to your website once in 30 days, and rank the site accordingly. Back then, webmasters were always nervous with the idea of their site having a downtime when Google would come to crawl.
Thankfully, we are past those times. Today, Googlebot is not once-in-a-month visitor. It accesses websites regularly. The Google’s Webmaster Tools still offers a notification suggesting if your website is down during the crawl.
There is a Google Webmaster help video pertaining to this subject. The video explains the scenario when Googlebot is not able to access the website, and how this influences the ranking of that website.
Matt Cutts, the face of Search Quality and SEO at Google, says that you need not worry if your website is down for a day, due to hosting issues or server transfer. He adds that such a situation will not negatively affect the search rankings of your website. On the other hand, if the downtime is for a rather long period, say a week or two, your search rankings may well be affected. This is because Google does not like directing users to a website that has been down for a long time as this creates poor user experience.
If a website registers downtime periodically, Google does make allowances. In this case, Googlebot will crawl the site after 24 hours to check if the downtime is still present or not.
Matt Cutts says that if you think the issue is not website-related, you can visit the Google Webmaster help forums to check for similar issues. He gave an example where the issue was at Googlebot’s end, wherein it was not able to fetch pages properly; and users received notices that their website was down. When browsed across the forums, it was found that several users faced the same issue, thus helping Google fix the problem at its end.
The feature on Google Webmaster Tools is simply to help users investigate if the issue is at their end.
If your website suffers a minor downtime (say 1 day), you do not have to worry, at least in terms of search rankings. Of course, that’s bad for user experience especially when you have a good traffic. Hosting issues, server transfers, these are a part and parcel of working on the Web.