You have been hacked to state it very clearly. Follow the list as posted by Mandville:
[ ] Ensure you have the latest version of Joomla
for your version of Joomla. Delete all files in your Joomla installation, saving a copy of the configuration.php file.
[ ] Review Vulnerable Extensions List
to make sure any 3rd party extensions versions used appear on the vulnerable list.
[ ] Review and action Security Checklist 7
Make sure you've gone through all of the steps.
[ ] Scan all machines with FTP, Joomla super admin, and Joomla admin access for malware, virus, trojans, spyware, etc. Checklist 7 contains a list or recommended scanners.
[ ] Change all passwords and if possible user names for the website host control panel. Change the Joomla database user name and password.
[ ] Use proper permissions on files and directories. They should never be 777
, ideal is 644 for files and 755 for directories. The configuration file can be set to 444 which is read only.
[ ] Check your htaccess for for any odd code (i.e. code which is not in the standard htaccess supplied as part of the Joomla installation).
[ ] Check the crontab or Task Scheduler for unexpected jobs/tasks.
[ ] Ensure you do not have anonymous ftp enabled.
[ ] Verify individually that any non-Joomla file such as but not limited to that will be placed back on the website such as images, pdf files, files for download, and other documents and files are valid and are supposed to be part of your website.
[ ] Replace the deleted files with fresh copies of a current full version of Joomla (minus the installation directory) you downloaded earlier. Install freshly downloaded copies of any extensions and templates used on the site. If the Joomla database user name and password were changed earlier, then make the necessary changes to the configuration.php file and upload a copy to the website. Upload any non-Joomla files that are necessary for your website. Only by replacing all files in the installation (including extensions and templates) can you be sure to remove the backdoors inserted and hidden in various files and directories
More detailed information can be found in the Security Checklist 7