Backing up your database is one thing, backing up all your files is another thing (a major, important thing).
wp-config is a file. It isn’t in your database. It lives in your FTP, in your WordPress folder. Go into the backup of your files and find the old wp-config, and use the exact same details from it, to make sure you got everything right such as username, password, and table prefix.
If you simply don’t have it, then anyway, just put the correct username, password, and table prefix in your wp-config and it should work. If these are filled in wrong, I think it should say “cannot connect to database”. If you’re not getting that error message you may have something else wrong, it’s hard to tell from what you said so far.
For the future, when you’re preparing to upgrade, do NOT delete wp-config, and do NOT delete your wp-contents folder. You delete everything else WordPress before upgrading.