Welcome to the forum.
OK, you have a Closed Path/Shape (visible in the Paths Palette). You wish to Fill that Closed Path/Shape with a selected Foreground Color, which appears in the Colors section of the Toolbar. What type of Layer are trying to Fill to? This can be very important, as if you are working in an Adjustment Layer, you will only get black, or white, or gray. If you happen to have a Channel (say from a Layer Mask), you will only Fill with black, or delete parts of the Mask with white.
Can you post a screen-cap of your Image with the Colors section of the Toolbar, the Layers Palette, the Paths Palette and the Channels Palette visible - just as they appear, when you attempt to Fill your Closed Path with the selected color? That will tell us a lot, and likely point out why you are having problems.