No video only audio on youtube on google chrome?

When I try playing videos on YouTube on Chrome, I can only hear the sound and a black screen appears where the video should have been playing. The captions and ads are still there. I'm having this problem since the last 3-4 days. I've tried re-installing Google chrome but the problem is still there. Everything works fine on Internet Explorer and Firefox. Also, I can play videos on other websites like Meta cafe.
re-installing flash player didn't work. also I found out I'm having the same problem with videos on facebook.