I signed up with Google webmaster tool. We get email from Google everyday reporting that Googlebot cannot access to the site.
If you see such error: Googlebot cannot access to the site, the most probably the reason may the following:
1. Domain name registrar has some issues with DNS.
2. Firewall may block some IPs that belong to Googlebot.
3. Robots.txt file is incorrect or missing.
4. Not properly configured DNS settings.
Also I advice you to try alternative way to check your website: https://support.google.com/webmasters/bin/answer.py?hl=en&answer=35658