If you have a backup of your server, then you can rollback to a previous version of any softeare installed on that server.
The steps described here will solve the problem on your system if you follow them cloesely, you should check them again.
1) are the mysql login details in /usr/local/ispconfig/server/lib/config.inc.php, /usr/local/ispconfig/interface/lib/config.inc.php and /usr/local/ispconfig/server/lib/mysql_clientdb.conf corrrect. Test every username password combination with phpmyadmin.
2) Does the ispconfig mysql database exist and does it contain the records for your websites, email addresses etc.
3) If 1+2 is ok, then redo the update by downloading the ispconfig 3.0.4 tar.gz to your server, unpack it, go to the folder
and there run