Webcam goes dark on Windows Live Messenger?

ok basically i was playing with my webcams with a friend on WLM, and i decided to see if my old one still works, so i plugged that one in and played for a bit. when i went back to my laptop's built in one that i always use it was too dark to see. why is it doing this? my laptop's an Advent something or other. don't ask me for all the technical crap cos it's likely to be something small that i can't find.