It means the webpage you were trying to reach on a website couldn't be found on the server.You can solve this by following the below steps/points:-
->Refresh the page-You can press Ctrl+F5 to force a refresh on the page. The error may appear just because the page wouldn't load.
->Clear browser's cache and cookies -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.
->Check the URL-Make sure that the URL input-ed correctly. This includes spelling of the URL, correct slashes, file extension of the page and so forth.
->Scan for Malware-There are some malware files that will force a 404 Error on websites. It's imperative to make sure your computer is clean and not compromised.
->Contact the Webmaster -Go ahead and send a message to the webmaster and let them know of the 404 error. If you are the website owner then contact your hosting provider for more information. If the server is encountering problems, it may return 404 errors.