sure its either u are beyond the lens minimum focusing distance and your camera coudl not AF the distance of the web because it is too thin, not much contrast to the background. so use manual within the focusing range.
On your lens, there should be a series of number. For example, look at your kit lens.
On the kit lens, the 18mm-55mm refers to the focal length of your lens. That means how much you can see in your picture. At 18mm, you can see a wide angled view, while at 55mm, you find that the view is narrowed to a smaller field. This is the zoom in your lens.
F3.5 - f5.6 refers to the minimum aperture of the lens. For example, when using the Av mode, when you adjust the dial to change the aperture, the smallest aperture number that you can get is f3.5 when your focal length is at 18mm. When you zoom in the 55mm, you find that the smallest aperture that you can use becomes 5.6.
If you look at the lens, towards the right, you see a small symbol that looks like a flower, followed by the number 0.28m/0.9ft. That is the minimum focusing distance. That means, if the object that you are trying to take is nearer than 0.28m, you cannot focus on the object.