Google was unable to connect to your server.
Use Fetch as Google to check if Googlebot can currently crawl your site. If Fetch as Googlebot returns the content of your homepage without problems, you can assume that Googlebot is generally able to access your site properly.
Check that your server is connected to the Internet. It’s also possible that your server is overloaded or misconfigured. If the problem persists, check with your hosting provider.
