Down term explained
Site down means the absence of any response from the server on which the site resides. The term means that the server was down or turned it off or overloaded with D-DOS attacks so that the server via IP address does not ping.
Most often the term refers to exactly the server. Less frequently the term refers to the position in the search engine results page.
If the site was down, you should contact the customer support of hosting company, if you are not a server administrator.