Yeah, now that we know it was shutter-priority, not full auto, and the ISO was set to the camera's minimum and not auto-ISO, it's clear that the situation was simply beyond the camera's ability to correct.
Shutter speed was locked because the camera was in shutter-priority mode. For the OP, that's S on the mode dial. That means the camera will use the shutter speed you've set, and adjust whatever else it's allowed to adjust to get the correct exposure.
So it adjusted aperture. It made the aperture the largest available (smallest number) to get the most light possible. Once it reached the lens's maximum, it had no more adjustment to make. f:4.0 was all it could do.
So then it looked at ISO, But ISO was set at 100, not auto-ISO, so there was no adjustment for the camera to make.
It did what it could, and took the picture anyway. You told it 1/3200, and at maximum aperture and the specified ISO, there wasn't enough light to get the picture.
Now the question is, why such a fast shutter speed?? 1/250 would have been my absolute fastest in that situation. You should have seen LO in the viewfinder to tell you the camera couldn't expose properly. Even if you'd set auto-ISO and let the camera salvage the image, it would have needed ISO 3200 or thereabouts, and I can tell you from having a D7000, you wouldn't have wanted that image.
The camera tried to tell you that you couldn't get the picture. use the information in the viewfinder. If you thought the camera was full auto rather than shutter-priority, then that was just a mistake, and one that is easy to make. It's easy to turn that dial accidentally when you pull the camera out of the bag or put it in, so check it every time you start shooting.