The most common way to hack a site is through php. If this person found an exploit in your forum software, they can make adjustments to any file they chose.
This is generally the kind of response you will get from all hosters. And, 99.9% of the time it's a correct assumption.
Just looking at your site I could tell you added a lot to it -- most likely the vulnerability is caused by one or more of the changes you did to the site.
Also, if you are giving different people access to your server/files to fix bugs and/or install products etc ... you are only putting yourself at risk. It just makes your job harder to find out who "hacked" your site.
Hacking a site could be done by adding a small script on a product, style, js, and/or flash file. This is why you should be careful of what you install on your site.
Check every file in your web account for files that do not belong there. If hackers got in they could have easily added or edited files.
You should check your addons for any suspicious code.
Change your config.php information (username/password). Most likely they didn't get this far but it's better safe than sorry.
Change FTP username/password.
My best recommendation is to start the site from scratch on a new hosting account. Also, limit OTHERS who have access to your server/files to 1.