Google couldn’t access your site because your server refused the connection. Your hosting provider may be blocking Googlebot, or there may be a problem with the configuration of their firewall.
Use Fetch as Google to check if Googlebot can currently crawl your site. If Fetch as Google returns the content of your homepage without problems, you can assume that Google is generally able to access your site properly.
It’s possible that your server is overloaded or misconfigured. If the problem persists, check with your hosting provider.