If you were using "Manual" mode, and you increased the shutter speed, the camera will not change the aperture setting to insure a correctly exposed photo.
Manual mode means you have to change the aperture setting.
If you were using "Shutter Priority" mode, and increase the shutter speed, the camera will adjust the aperture to correctly expose the photo (if it can).
If the aperture is at maximum (f2.8) the camera can not adjust it further, and you will get a dark photo.
The camera may adjust the ISO to provide more light (effectively), but you would need to verify that with the User's Manual.
You can always increase the ISO setting yourself.
It would be helpful to know for sure what mode you were using.
I've just bought an FZ5 and i want to take photos at the football (Aussie Rules) at an indoor stadium under stadium lights. I took the camera to a rugby game recently, but could seem to get steady photos when the players were moving. when i tried to set a higher shutter speed the photos were coming out too dark. I wasn't using a tripod.
Is there some setting that's good for those conditions?