On your site files for ubb you have a includes folder.
In that folder is a filename
It holds most all the settings for your site that you enter in the ubb control panel.
You can look and edit it from a text editor. Like notepad or Notepad ++ do not use wordpad or word.
I appears that the file got corrupted or deleted somehow
If you have any type of current backup of the script files after installation then upload that to the site to overwrite the file.
You might even find your custom body on load.
As a precaution I would ftp to the site just to make sure the config.inc.php file exists before you start anything.
And if you find a backup config file view the contents to see if it would be better to use it or let it be.
If you don't have a usable backup then below still applies.
Then in ubb control panel
Methodically review every single section and every single entry field one at a time edit what needs to be corrected and save/update.
Even if it looks correct with no edits save/update.
In the future I would at the very least make regular backups of the includes folder.
This is not a substitute for a full backup of files and database
You can find a dummy config file at ubbwiki.
to compare to what you should have. https://www.ubbwiki.com/archive/fetch/1/ubb-threads-v7-config-file.html
It will have different data than what you need but it will contain all the fields blank or not for what your forum needs.
Only you will know what your settings should be.
Every site is different.