Modx Evolution 0.9.5 - Document content is blank though the page comes up in the preview section

I can't edit anything in a site running Modx 0.9.5. The document content for a page is completely blank. But the site is running fine and the preview from the page comes out ok. I will appreciate any help to resolve this issue.

Could be an update of the server / php.
Or the site has been moved to an other server without knowing.
Maybe you can do a backup and update to 1.0.15?
Also check if there are suspicious files, check last changed dates etc.
You can do it.