Why wouldn't my PC speakers function so suddenly?

I wake up one day worrying what's going on with these speakers. I selected a track from Windows Media Player, but it wouldn't play and it would instead select random songs but not playing them anyway. After various attempts of playing a file, WMP had this dialog box pop on my face, "Windows Media Player cannot play the file because there is a problem with your sound device. There may not be a sound device installed on your computer, it may be in use by another program, or it may not be functioning properly." I tried Winamp but it's just the same, I tried the net, particularly Youtube, but I hear no sounds at all. I tried my mp3, it works. Please help. It's so sudden.