What might cause shutter lock up on Nikon D90 after taking others with no problems?

Inside very well lighted gym, other photos taken but when shooting a small group in same location the shutter locked up. ISO/ASA 200, f6, auto focus, flash off as with other photos taken and the shutter locked up. Had the same problem when shooting a flower outside on sunny day: ISO 200, f3.5 to throw background out of focus, set on auto focus. Shutter locked up. Battery was charged, lens on correctly.

With any camera, there's a minimum focus distance each lens has. Depending on your lens, your minimum focusing distance may be 1" to 3ft. When you are in autofocus if you are too close for the lens to properly focus, the camera will not let you take a photo.

Usually in the viewfinder, there will be a red box when you depress the shutter half way down, and will turn green when the camera is in focus. You will usually hear a beep confirmation tone as well. And in the view finder, the focus assist should show a round circle (rather than a left or right pointing arrow). If you get none of these conditions, your camera is not in focus, and it will not let you take a photo.

Also in low light, the camera may not focus properly. The camera (all cameras) require a minimum amount of light before the autofocus system will work. There's a focus assist lamp that you can turn on or off which will help illuminate the scene for better autofocus.

I suspect nothing is wrong with your camera, but you are operating beyond the capability of your camera and lens. If you buy any other camera, you will have the same results.

If you want to shoot flowers close up - you need to buy a macro lens.

You can confirm whether or not the camera is preventing you from taking a photo if you are too close for your lens, or if it is too dark - by flipping the Automatic/Manual focus switch on either the lens or on the camera to manual. In manual mode, the camera ignores whether or not the camera is focused.

If you can take a photo in manual focus mode, there's nothing wrong with the camera. If you can't then there may be a problem with it.

However even in manual mode, if you try to focus too close for the lens, the image will be blurry.

While Nikon makes some of the best autofocus systems available, there will be situations where no autofocus system will work. That is why lenses have manual focus capability.

Refer to your manual for the basic techniques of autofocusing.

I owned a D90 for 4 years before giving it to my son. It has a very good focusing system.