MacBook Air camera no longer working

Yesterday my Skype was working perfectly , with good video. Today I could see the caller perfectly but she could not see me. At this stage I still had a video button on screen but it didn't do anything. I tried reloading Skype and now I don't even have a video button. It is as though Skype no longer recognises my computer's camera. I have the latest up to date software on my mac - version 10.9.4. Now I find that Photo Booth says "there is no connected camera". Any suggestions?