Black screen on Youtube

[jellybean2]

Hello everyone,
As you will rapidly understand, I am clueless about computers. I have a secondhand Macbook, and it used to work fine for viewing YouTube, but now I just get a black screen and the video doesn't play. The operating system is Yosemite.I can get still frames by clicking on the timer bar below, but no sound either. Any idea why?
Thank you,
Jellybean

 

[vembutech]

Does it work in all other browsers like chrome , firefox ??