Hi everyone. Today I tried to watch a video on my computer, and there was no sound. So I clicked the volume up button, and it doesn't change the level (there's a small "no" sign, the circle with the line through it, under where the volume level should be). In the menu bar, the volume control is now a light grey and I can't use it. Same with in system preferences. Any ideas on how to get volume to start working again or why it stopped? Thanks for the help.