Well, I didn't back up the files from 7.6.2 (stupid me). There was no config-bak.inc.php file in the /includes directory.
I still had the back up from the previous version and uploaded config.inc.php.
The forum is working.
I also uploaded the other files from the backup (header, footer, intros to each forum, etc.). Looks like it is all back to normal.
We have been using the UBB Forum software on our site since 1998. 20+ years!