The first thing you should do is simply refresh. You can press Ctrl+F5 to force a refresh on the page. The error may appear just because the page wouldn’t load.
If you are able to visit the page from another device, such as your phone, clearing the cache and cookies will usually solve the problem.
Make sure that the URL inputted correctly. This includes spelling of the URL, correct slashes, file extension of the official website page and so forth.