This error could severely impact your blog if you have many members to login to your admin page. The error would not allow you to see the login screen and thus is a major issue.
If login page does not show up or shows a 404 error, MySQL database need to be opened through your hosting backend – preferably through PhpMyAdmin panel. Using this panel, try to open the database where your WordPress data is hosted. Browse through the tables and select the table with the name ‘wp_options’. Edit the row where the option_name is ‘siteurl’. You need to change the URL in the option_value. If you edit it with the correct URL, then you can open the site and work with it. Usually an incorrect URL causes this 404 error message.