Answered Content of all resources lost in Manager

I'm still running Revo2.3.3 and prepare to upgrade to latest version.
I have upgraded all my extras (Babel, TinyMCE etc).
I changed PHP version from PHP5.5 to PHP5.6 - without logging out of Manager first.

Now ALL my resource pages appear to be empty. Under "Settings" the "Parent Directory" is empty.
Reverting to PHP5.5 didn't help.
Clicking "View" however is still showing the page properly.

I'm desperate for hints about what might be broken.

Have you checked the MODX Error log and the PHP error log? If the resources are still working in the frontend, it sounds like it's just running into an error loading them in the manager, and they're not actually lost.

I agree with Mark, the resources are almost certainly still there. There are some goofy things that will trash the display in the tree. One possible cause is dragging things around in the tree. In very rare cases a resources can become its own parent or two resources can become each other's parent. That crashes the code that draws the tree. A resource with an invalid parent can also cause trouble.

SiteCheck is one option for checking the validity of resources, but you can also check them yourself in the database.