How can I fix my Microsoft Webcam?

I have a newish webcam and when i first bought it I was able to see myself and hear myself when talking to others on skye and so on. But I believe there is a problem as when the little blue light turns on (meaning the webcam is turned on) all I see is black when videoing and trying to take a photo , the strange thing about it is that my voice can be heard but you can't see the picture. Please help.