If you are using static HTML pages with no user input, then somebody managed to gain access directly to your files. Re-uploading your files and changing your cPanel password (and making it a strong password, and keeping it to yourself) ought to clear that up. If you have links to your site elsewhere, such as in forums on the web, and you're using the same password in more than one place, then it can be easy to break in (you'd be amazed at how many sites store passwords in plain text or in an easily-cracked unsalted hash format).
You may also be using a script (JavaScript or PHP) that you didn't write yourself, and which may come with its own built-in exploit, so you might want to go over things before you upload more than an "under construction" page. Free scripts can be nice to have, but only if you know exactly what they're doing -- or if they are coming from a known-trustworthy source (like linking jQuery from the Google APIs site, for example).
Please note that images (JPEGs in particular) can also be exploitable -- there is a lot that is allowed in an image file that has nothing at all to do with creating a picture. If you are using images you didn't create yourself, you might want to run them through something like
PureJPEG to remove anything that isn't necessary to displaying the image.
