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I was updating a site and I got the following error message:
Fatal error: Undefined class constant 'PRODUCT' in /home/sitename/public_html/administrator/modules/mod_version/helper.php on line 39
Now the site will not load the administrator interface. I saw this https://github.com/joomla/joomla-cms/issues/9515 but its not offering a real solution.
The front of the site is still working. Any suggestion would be most helpful!
As a possible quick workaround - connect via FTP and go to /administrator/modules/mod_version/ rename it to something like mod_version-disabled.
See if at first place resolves the issue with the backend. Generally something didn't went really well with the update and some files might not have been copied/replaced right.
If renaming the above module's folder name, gives you admin access back, then you could try re-installing the Joomla 3.5 by downloading the package from Joomla.org.
If you are still facing problems, you could and should grab a fresh copy of Joomla (unzip the full stable 3.5 package) and manually replace certain directories - but take care to not replace directories that contain additional non-core content.
I mean that usually you can fully replace the includes, layouts folders but for folders like components, modules etc you need to only replace the sub-contents that is part of the core distribution.
I would imagine you could update manually by downloading the update package and doing it by uploading the necessary folders and files rather then using the extension manager.
