Help me, Please!

Suddenly,my PC can't play online video in youtube.com and showed following message:
Hello, you either have JavaScript turned off or an old version of Adobe's Flash Player.

I reinstalled java6 update 13 after removed previous one and installed latest
Flash Player. It still doesn't work!

I didn't turned off javaScript, I don't know what it is.

What do I need to do?

Thanks for your help and time!