how to solve [this site can t be reached the connection] this error?

I attached the error page while loading. some times it shows. cannot find the error.

That error could be any one of the following

• The web server isn't running
  
• The web server doesn't understand the URL - for a lot of different reasons
  
• The network issue e.g. it's down or misconfigured, or a firewall is blocking the site,