I opened my macbook this morning and used it for a while until I needed to change the volume level. I pressed F4 to turn it down, but for some reason it won't, and I have a "not allowed" symbol under the volume notches. It will either be full blast or mute. I tried adjusting it from the toolbar but it won't move there either (and it's grey in color). I tried repairing permissions and that did nothing. How can I fix this?