Look for a sites database table in your database and delete the old site that you messed up and replace it with the one from your backup
Hi mokito,
none of these plugins will enable multisite. Have you ever used multisite before? If you want i could give you my multisite set-up that you put in your details for your server (you do this unless you want me to) and you are done (in the config of the wp-config.php and .htaccess i will do for you if you dont then i would advise http://codex.ww.wp.xz.cn/Create_A_Network however i should say that the plug-ins you are looking for are very useful if you do decide to go for a multi-site.
