Recently I had been having some issues with Youtube videos not working. In Firefox they would play the audio but no video. In Chrome they would freeze up right away and not play at all. I tried updating flash, updating drivers, clearing cache, restarting, you name it, but for some reason the issues continued.
I noticed that it seemed to happen only on videos with advertisements, as other videos without ads were playing ok. I also noticed that if I disabled Adblock Plus, the videos would play an ad and then they would load ok, so I figured it may be an Adblock Plus issue. A little research showed that others were also having this issue.
My Adblock was up to date and it was still causing the issues which was pretty annoying. Instead of trying to modify it manually, I just decided to disable it and install another ad blocker called uBlock. It does pretty much the same thing and so far it has been working out pretty well. It still blocks the ads and now the Youtube videos are playing without any issues.
It will take some time to get a full evaluation of uBlock, but as of now, I would recommend trying it out if you have been having issues with Youtube videos not playing and you have AdBlock Plus installed. It is available as a free add on or extension for Firefox and Chrome.