There are several reason why content in your site may appear missing after activating WPML. These reasons include incomplete language tables or other plugins adding content without including language information.
WPML's troubleshooting page offers you several actions, which can help resolve missing content in your site. To get started, go to WPML → Support → Troubleshooting. The troubleshooting link is located at the bottom right of the page.
If you created content while WPML was inactive, or if other plugins create content and bypass the WordPress API, this content may not have language information and will not display for any language (both in the front-end and back-end).
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.
WPML stores the available languages in a database table. That table might have missing entries due to incomplete initial installation of WPML.
Click on clear language information and repopulate languages to recreate WPML's table of languages. If you have created custom languages, they will be lost during this operation and you will need to recreate them.
If all else fails and as a last resort, you can completely reset WPML's language information and start over. Use the Reset section to permanently delete WPML's configuration and the language information for all content.
Check the check-box and click on the button. This operation cannot be undone. It will delete all language information from your site, getting it ready for a fresh install.
Then, go to the Plugins page and activate WPML again.
If problems persist, please also enable debug mode and check for any PHP errors and warnings during the activation.