Firstly, disable your plugins. This action helped me very well, i went to my cpanel > File Manager > Wp Content > Plugins and disabled all the active plugins then i went back to my Wordpress dashboard (guess what?.... it worked). I then began to install them one by one until i discovered the bad egg.

If your is not solved, read more solutions and tips @ http://arewatech.com/2012/11/how-to-fix-404-not-found-error-on-wordpress.html