I love using a Mac. But sometimes things don't 'just work' the way you expect them to on Apple's finest. This morning I was checking a video on YouTube using Safari and I couldn't play it. I couldn't play any video. While I could see the thumbnails, the video area itself was an unrelenting black screen which didn't respond and was to all intents and purposes dead. I pondered for a moment whether the latest system update could have affected it, but then decided a quick search was in order before I started muttering and growling. Usually someone else has had a similar problem and found a solution, which was indeed the case. So here is a fix I found on the Google Support Forum:
1) Right-click (or control-click) on the Safari icon in "Applications" folder.
2) Choose "get info" from the contextual menu.
3) Select "open in 32-bit" in Get Info, and then close the pane.
4) Re-launch Safari. The videos should play now.
This fix worked for me - good luck if you're experiencing a similar problem!