If you're talking about an embedded YouTube video, they recently changed their embed code as well so Browser & version would be helpful to know.
They went from:
<iframe width="560" height="315" src="http://www.youtube.com/embed/E6YuYHJ-npE?rel=0" frameborder="0" allowfullscreen></iframe>
to:
<iframe width="560" height="315" src="//www.youtube.com/embed/E6YuYHJ-npE?rel=0" frameborder="0" allowfullscreen></iframe>
That removal of the http: has caused issues for some users with various configurations, you might want to try that and see if it solves your problem.