If you have posts or pages that are now returning a 404 error, don’t panic (yet) most of the time your posts or pages are still there and you just need to update your permalink settings to fix this issue.
This is very common with themes that use custom post types. Whenever you activate a new theme that uses custom post types or if you update the slug of a post type in the code then you must reset your permalink settings.
- The Fix: Go to Dashboard → Settings → Permalinks and click the Save Changes button.
If this doesn’t work then you might have to update your .htaccess file manually.
Read more about using WordPress permalinks.