A 404 error is an HTTP status code that means that the page you were trying to reach on a website couldn't be found on their server.
Technically, an Error 404 is a client-side error, implying that the error is your mistake, either because you typed the URL in wrong or the page has been moved or removed from the website and you should have known.
Another possibility is if a website has moved a page or resource but did so without redirecting the old URL to the new one. When that happens, you'll receive a 404 error instead of being automatically routed to the new page.
Retry the web page by pressing F5, clicking the refresh/reload button, or trying the URL from the address bar again.
Often times the 404 Not Found error appears because the URL was typed wrong or the link that was clicked on points to the wrong URL. Check the URL for errors.
Move up one directory level at a time in the URL until you find something.
For example, if www.web.com/a/b/c.html gave you the 404 Not Found error, move up to www.web.com/a/b/. If you get nothing here (or an error), move up to www.web.com/a/. This should lead you toward what you're looking for or at least confirm that it's no longer available.
Tip: If you have moved all the way up to the website's homepage, try to run a search for the information you're looking for. If the site doesn't have a search function, try navigating to the page you want using category links to dig deeper into the site.
Search for the page at a popular search engine. It's possible that you simply have the entirely wrong URL in which case a quick search should get you where you want to go.
If you do find the page you were after, update your bookmark or favorite to avoid the HTTP 404 error in the future.
Clear your browser's cache if you have any indication that the 404 Not Found message might just be yours. For example, if you can reach the URL from your phone but not from your tablet, clearing the cache on your tablet's browser might help. You might also consider clearing your browser's cookies, or at least the one(s) involved with the website in question, if clearing the cache didn't work.
Lastly, check that the page exists and has not been removed, or the page name/URL has not changed or been changed by one of your staff or a developer. If you have staff working on your website for you, it may be that it is meant to be redirecting but the redirect has not been put in place.