Hi,
With a strong light and a magnifying glass visually inspect the USB port(s). Check that the pins are all there and that they are straight and parallel with each other and that there is no dust or lint in the port.
It could be that there is a short circuit between two of the pins that is "calling" the USB controller but obviously as nothing is connected it doesn't know what to connect.
If the visual inspection doesn't show anything plug in a known working USB device (not self powered) which has power indicator light e.g. optical mouse. If the light works then the USB power is OK, and if the device doesn't work then the data lines are faulty. (USB pins are +5V (VCC), data-ve, data+ve, Eth looking from the left as you face it)