There is a possibility where your microphone can be muted, can go to audio devices and check there by going to your control panel or on the bottom right hand corner, if that isn't the problem, there might be another way, I do not know if you have this on your computer but most people have the RealTek audio, can check that to see if your microphone is muted, or you can go in to your skype audio settings and see if that works