May be grey box is confilicting with fancy box. Remove the greybox scripts and try. it will work
<script src="//code.jquery.com/jquery-1.8.0.min.js"></script>
<script type="text/javascript" src="fancybox/jquery.fancybox.js?v=2.1.1"></script>
<link rel="stylesheet" type="text/css" href="fancybox/jquery.fancybox.css?v=2.1.1" media="screen" />
<!--
<script type="text/javascript">
var GB_ROOT_DIR = "greybox/";
</script>
<script type="text/javascript" src="greybox/AJS.js"></script>
<script type="text/javascript" src="greybox/AJS_fx.js"></script>
<script type="text/javascript" src="greybox/gb_scripts.js"></script>
<link href="greybox/gb_styles.css" rel="stylesheet" type="text/css" media="all" />
-->