So many times its happens with my mac that Skype, Chrome other applications are not able to access built in webcam. After googling i came up with following solution.
Usually it’s not a macbook problem. It’s software problem.
open a terminal and type
sudo killall VDCAssistant
This will trigger a restart of the camera’s software. Fixes the problem 99%. Skype and other software that uses the camera might also need reopening.