If you have posts that are now returning a 404 error page, don’t panic (yet) most of the time your posts are still there and you just need to update your permalink settings to fix the issue. This is very common with themes that use custom post types. Whenever you activate a new theme that uses custom post types you should reset your permalink settings.
Posts Showing 404 Error Solution
The simplest fix is to just go to Settings > Permalinks and click the save button. Usually this will do the trick. But if this doesn’t work you might have to update your .htaccess file manually.
# 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
