Googlebot could not access your site because we couldn’t communicate with the DNS server, or because your server had no entry for your site. This could be because the server is down, or because there’s an issue with the DNS routing to your domain. Please check back later. If this problem persists, check with your DNS provider (often, but not always, your web hoster).
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.
While most DNS warnings or errors don’t affect Googlebot's ability to access your site, they may be a symptom of high latency, which can negatively impact your users.
Websites can be configured to respond to all subdomain requests. For example, the website at example.com can be configured to respond to requests for foo.example.com, made-up-name.example.com and all other subdomains. This makes sense for some sites—for example, in the case where a site with user-generated content wants to give each account its own domain (http://www.example.com/username). However, in some cases this kind of configuration may cause content to be duplicated unnecessarily across different hostnames, and it may also affect Googlebot’s crawling.
To minimize problems in wildcard DNS setups, either configure your website to not use them, or configure your server to not respond successfully to non-existent hostnames, either by refusing the connection or by returning an error HTTP status code, such as 404 or 500.