If you believe your Web site was attacked, do not create yet another oh-so-boring post in the Joomla! forums with the title, "Help! I've been hacked." This tells us nothing of importance. The vast majority of compromised sites were not setup correctly or were using obsolete versions of Joomla! or third-party extensions. This is what you need to investigate.
If you discover a real vulnerability, publishing the information could put other Web sites at risk. Instead, report possible security vulnerabilities to the Joomla! Security Task Force.
Follow a logical and rigorous recovery process
Know the important steps to follow when your site has been compromised. Once your site has been cracked, there are few shortcuts. (FAQ)
Reset your administrator password
Many attackers take pleasure in locking you out of your site. They often do this by changing your administrator password. If you are locked out, don't panic! There is a simple procedure for resetting your administrator password. (FAQ)
Find exploit attempts using the *NIX shell
Know how to check for suspicious and/or modified files. Know how to check the raw Apache logs for suspicious activity on your site. (FAQ)
If you discover a vulnerability in Joomla! core files, report it here.