As I suspected: empty table prefix.
Take a copy of your config.php (just for backup reason)and then edit the original >>> $table_prefix = 'bb_';
Delete /installer and/production from /cache, clear browser cache and then try to access your board.
BTW, take a copy of that database (in case anything goes wrong - you'll be able to restore that database).
If you get your board back up and running that way : take a full backup and then update using files downloaded from phpBB.com (no installatron!!)