Hacked

My site http://www.bigskycoast.com has just been hacked by somebody calling themselves Jago-dz, the whole of the site is missing from my server with just a script that says –

<?php
if ($_GET['randomId'] != "41zQHxvTFivwijaJc7tma3Hj2_eKJGoUn5HeyDbTQ464uuRw8Ynj4YIm8MGkKPJ_") {
    echo "Access Denied";
    exit();
}

// display the HTML code:
echo stripslashes($_POST['wproPreviewHTML']);

?>

left on the server with other sub domains of the server looking intact but throwing up a 404 error page.

Any ideas what to do, who is behind this and how to protect against it in the future.

Grrrrrrrrrrrrrrrr

Holmey