Is it possible to get a website up and running like the way it was before if everything has been deleted but the SQL DB is in place (We are talking about SMF, VB forums and Wordpress blogs here) IT
Yes, you would have to reinstall all the components and reconfigure them with appropriate paths and SQL info. Although if you don't have the design (css and templates) it won't look the same.
Well, of course if you have backups, it will be a lot easier but otherwise, it would still be possible. Some scripts just require an upload of the script files and then just updating some config.php file which contains the basic information such as database credentials. Some has the 'update' or 'upgrade' script which you can run to get all the permissions set up. The only thing is that all the customization you done to the scripts and settings (If they are not stored in the database) would be lost.