Retry the web page by pressing F5, clicking/tapping the refresh/reload button, or trying the URL from the address bar again.

Hard reset the browser:
​For Windows
* Hold down Ctrl and click the Reload button.
* Or, Hold down Ctrl and press F5.
* Open the Chrome Dev Tools by pressing F12. Once the chrome dev tools are open, just right click on the refresh button and a menu will drop down. This menu gives you the option of doing a hard refresh, or even clearing the cache and do a hard refresh automatically.
​For Mac
* Hold ⇧ Shift and click the Reload button.
* Or, hold down ⌘ Cmd and ⇧ Shift key and then press R.

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.

Search for the page from 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
(Tip: Change it to Google DNS 8.8.8.8 alt 8.1.1.8 or Cloudflare DNS 1.1.1.1 alt 1.0.0.1)

Clear the DNS
​For Windows:
Press Windows+R to open the “Run” box. Type “cmd” into the box and then press Ctrl+Shift+Enter to run the command as an administrator. Type "ipconfig /flushdns " and press Enter. Type "netsh winsock reset" and press Enter. Restart the computer.
​
​For Mac:
Open Terminal> sudo killall -HUP mDNSResponder (Mac OS X Yosemite and later)
> sudo discoveryutil mdnsflushcache (Mac OS X Yosemite v10.10 through v10.10.3)
> sudo killall -HUP mDNSResponder (Mac OS X Mavericks, Mountain Lion and Lion)
> sudo dscacheutil -flushcache (Mac OS X Snow Leopard)