Webcam not working in AMSN on macbook

It always did work and then like a month ago it stopped working all together. At first it just wouldn't let me recieve peoples but would send mine as a white screen now it won't let me send or recieve. It just cancels it automatically.

Really stuck on what to do as all i found online is very technical explanations about routers which i don't really understand, can anyone help me please?