As a possible quick workaround - connect via FTP and go to
/administrator/modules/mod_version/ rename it to something like
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.