What does it mean when the server of a website can't be found or..?

It typically means the server is not resolving. Either the URL is not pointed to an IP address or there are no files on the server. Sometimes it's also an issue with your internet. I always check out another webpage that is dynamic, something like yahoo or your favorite blog. If the latest articles pop up then your internet service is fine and there is typically something wrong with the previous site you were visiting