I sometimes get a message that I either don't have Flash, OR that Javascript is turned off. If YouTube tries to set its cookie and fails, it won't play, so I have to 'clear my cache' in order to allow my browser to get rid of the setting that YouTube doesn't like.

I have no idea if that is affecting you, though. If you tell us which video, we can see if it plays, just to be sure it hasn't been removed.