In case your WordPress website comes loaded with WPML plugin, this can be one of the reasons of the problem. If you created content while WPML was inactive, this content may not have language information and will not display for any language (both in the front-end and back-end).
To resolve the missing content in your site, add language information to the content that doesn’t have languages. Go to WPML -> Support -> Troubleshooting, look for the “Clean up” section and click on Set language information. WPML will put all content without any language in the site’s default language.