Below we will focus on the two most common reasons for a website to display a "404 Not Found" error message.
1. The content you are trying to reach is actually not uploaded
If a link on your website leads to a "404 Not Found" error, we strongly recommend that you check if the link is correct and if the page that is supposed to be reached is uploaded to the right location. The right location is usually the public_html directory for your main website or a subdirectory within the public_html directory for a subdomain or other folder.
2. Some SEO/SEF (Search Engine Friendly) functionality of your application is not properly set up.
Many customers use open source applications like Joomla, Drupal etc. on their web hosting accounts. Some of these applications require that a special functionality is enabled in order to generate Search Engine Friendly URLs. Very often enabling this functionality includes some editing of the .htaccess file. A common case of a "404 Not Found" error is resulting from some incomplete or wrong rules in the .htaccess file or another mistake in the setup of the SEF functionality.
So, if you start seeing the "404 Not Found" error after you have tried to enable the SEF functionality of your application, we would advise you to check carefully if you have set it up properly.
If you cannot find the reason for the "404 Not Found" error page, you should ask your host for assistance.