This is one common solution if you're getting a "404 Not Found" error when visiting your Joomla administrator area. The problem often lies with a good and useful product called JSecure. You normally have to remember a specific URL that was set in the administrator area. For example: mysite.com/administrator?mysecretcode. This stops people from trying to guess your login, but if you forget your secret code ...
Do you see this screen?
If you do, the problem is JSecure. Have you really forgotten your secret code? If so, read on ...
Login to Your Hosting Account
Or you could go via FTP. Essentially you just need a way to get to your files.
Find Your File Manager
Every hosting company should give you access to your files, although the access may be in different places with each host. This is an example from the popular CPanel software.
Browse to /Plugins/System/
You need to access the /plugins/ folder than then the /system/ folder.
Find JSecure.php
Rename the File
Just give jsecure.php a different name and save it. Next time to try to access your site, the 404 error should have gone.