no longer have any sound

I was trying to Skype phone and each time I called, the call ended. No matter what number I was calling it said I could not reach that person. Lately the volume has not been showing up in the bar and I go to System preferences and turn it on but when I tried to do that tonight I was unable to. So I have no sound. My System Preferences Sound "output" shows that I have "no output devices" I have no idea why this happened.