The light on the back of the camera head will only be illuminated when the camera is streaming video.
If it isn’t lighting up, fully quit the camera software you’re using so that it does not show up in your Dock or Taskbar. Then try opening it again, making sure that the camera isn’t in use in any other applications which might be running in the background. USB cameras will only work in one application at a time, and sometimes even web browsers can reserve the video stream with certain plugins installed.
Please also make sure that the camera is directly connected to your computer, not to a USB hub or through a port in the keyboard. If you change the USB port the camera is connected to, please make sure that the software is fully quit as above and then reopen it after reconnecting the device.
HUE Intuition users don’t need to close the software. Simply changing to another tab and then back again will reset the camera connection.