HELP! Music Files Stopped Working

So I am at my computer putting a Wod Document together listening to Yahoo! Music, when all of a suddent the sound stopped. I stopped the station, and put on another, same thing, nothing, but the timer continues to add up. So, I turn on Windows Media Player, no big deal. Well, I recieve an error on ALL my music files, that access is denied. My sister had this problem with her system, and I thought she screwed something up, but is it a Windows problem?

I am running Windows Vista Ultimate, Realtek drivers that are up to date.

Can someone help?

~Jon