HTTP
404 Not Found
What it means
The server you reached could not find a resource at that address. The connection itself worked: the name resolved and the machine answered. It then said the path is not there.
Where you usually see it
Mistyped addresses, old bookmarks, and links that were never updated after a page moved. On a site you run, it also appears when a rewrite rule or a missing file does not send the request where you think.
What to try first
Check the address for a typo or a missing part of the path. If you followed a link, try the site's home page and look for a current route to the same content. If you run the site, confirm that the file or the route exists at that path and that the web server is allowed to read it.
When to stop
A 404 is not a reason to reinstall a browser or to change DNS. If many unrelated sites return 404, you are probably looking at something else, such as a captured DNS response or a filter page dressed up as a 404.