You can see the problem if you check out the iframe with (for example) Chrome's web inspector:
<iframe id="fancybox-frame" name="fancybox-frame1352465749626" frameborder="0" hspace="0" scrolling="auto" src="undefined"></iframe>
The src attribute is undefined, whereas it should read http://www.youtube.com/watch?feature=endscreen&NR=1&v=yHocNtrD3FQ
If I were you, I would make a bare bones page with one FancyBox video on it (which hopefully will then work), and compare this with your current page.
If you can do this and post a link to a barebones page with a working FancyBox video on it, I don't mind taking a look.