Why is the volume on my computer not working?

Just the other day I went to you tube and was starting to watch a Miley and Mandy video and my sound didn't work. It hasn't worked since then. There aren't head phones plugged in and the sound is all the way up, and its not muted. What could be wrong? Its a brand new computer, I got it in Febuary...or December.....so its not brand new, but its not old either. Its a newer computer, there that sounds right.