It sounds like your camera is not able to confirm focus and therefore refuses to take the picture.

The default behavior for most DSLRs, including your D3100, is to confirm focus before releasing the shutter. The lamp on the front of the camera is lighting in an attempt to assist the autofocus system. This would indicate you are attempting to shoot in very low light.

Possible reasons why your camera can not achieve focus confirmation:

• There's not enough light in the direction your camera is pointed.
• There's not enough contrast in the direction your camera is pointed.
• Your subject is closer to the camera than the lens' minimum focus distance.

For a more detailed look at this possible issue, please see: Why does my Nikon D3100 not fire the shutter in autofocus mode?

Another possibility is that you have the camera's drive mode set to 'self-timer'. In that case, the lamp is illuminating to confirm that the countdown to shutter release has begun. If you wait either 2 or 10 seconds does the camera attempt to take the picture? If the camera can't confirm autofocus during self-timer operation, please see above.