Ah! Embedded YouTube videos! Got it now. You say you can't watch a YouTube video on this site, but anywhere else you can. To be honest, I genuinely have no clue why that's happening to you.
Firefox 37 (the most recent one) and nearly every version of Chrome play YouTube videos with HTML5 instead of Flash. Perhaps, for some reason, your PC isn't working well with HTML5?
Try checking this page:
https://www.youtube.com/html5 -- If it says
"the HTML5 player is currently used when possible" then HTML5 is working. Firefox is more problematic for me when playing HTML5 videos, and if that might be the case for you, you can try to
force it to run Flash on YouTube. If Firefox is what you're using, this add-on might be useful:
https://addons.mozilla.org/en-US/firefo ... eo-player/Maybe that will help.
My primary browser is Chrome and I rarely ever have trouble playing videos on any site with it. So I don't really know the exact cause of your problem. Sorry, but I hope at least some of that info is useful.