The symptoms of this error is that when a user visits a single post on their site they get a 404 page – not found error.
The user can browse all other sections of their site including the admin area. The most common cause of this issue is permalink settings in WordPress.
To solve this issue a user would need to reconfigure their permalinks settings or manually update their rewrite rules.
Solutions:
a) Update your permalink settings
b) You can also manually add this code in your .htaccess file:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
http://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-posts-returning-404-error/