How to install the older version of wordpress

You can download the archive version of WordPress that you desire and reinstall at your hosting.

https://www.ads-software.com/download/release-archive/

Thanks, I have downloaded 3.4.1, but how can I install it.

Follow the tutorial below for manual installation but be sure to backup your database and codes before you proceed. (Just for precaution purpose)

https://www.siteground.com/tutorials/wordpress/wordpress_manual_installation.htm

If the only thing that ‘made your posts disappear’ was upgrading WordPress core, then either:
1. It failed to correctly update.
2. Your theme, or a plugin no longer works with core code.

Re-installing the upgrade should fix scenario 1. Disabling plugins (or theme) should fix scenario 2.

It is best to keep up to date, and use only themes and plugins that work with the latest version.

After I disable all of your plugins, and then test my site. It has the same question.

All my post and pages are missing.