I tried to do the upgrade to the latest version for our community forum - and the upgrade failed. And now I can't move forward or use the forum. How do I resolve the failed upgrade to get the community forum back?

It seems something stopped on your browser there, rather than in the upgrader itself. If that happens, you can get back to the upgrader from /admin/upgrade and continue from there. I have completed this off for you now, so your site is up to date and ready for you again.