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.
404 Not Found error messages are frequently customized by individual websites. You can see some of the more creative ones in Tim Fisher's 20 Best 404 Error Pages Ever slideshow.How To Fix the 404 Not Found Error
Retry the web page by pressing F5, clicking the refresh/reload button, or trying the URL from the address bar again.
The 404 Not Found error might appear for several reasons even though no real issue exists, so sometimes a simple refresh will often load the page you were looking for.
Check for errors in the URL. 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.
Move up one directory level at a time in the URL until you find something.
For example, if www.web.com/a/b/c.htm 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 Google or Bing 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.
Change the DNS servers used by your computer but usually only if an entire website is giving you a 404 error, especially if the website is available to those on other networks (e.g. your mobile phone network or a friend in another city).
404's on an entire website isn't particularly common unless your ISP or government filters/censors websites. No matter the reason, if it does happen, giving another set of DNS servers a try is a good step to take. See my Public DNS Servers List for some alternatives and instructions on doing this.
Finally, if all else fails, contact the website directly. If they've removed the page you're after then the 404 error is completely legitimate and they should be able to tell you that. If they've moved the page, and are generating 404's instead of redirecting visitors to the new page, they'll be happy to hear from you so they can go fix it.