Is there just the one socker? If not, do the others work? And are you sure they are for the antenna? There isn't a Foxtel box lurking on the side of the house? It's unfortunately pretty common when getting a Foxtel connection to abandon the antenna based system.
You can also visually inspect the mast from the ground to see that the cable does actually go up to the antenna. If there appears to be a box on the mast, then you probably have a mast-head amp; I'd then go looking for it's power supply (it should be plugged into one of the sockets) and check it's powered on.
Past that, you're going to need a contractor. They will have devices to look at the raw signal level (you can't trust the numbers on set-top boxes or in digital TVs, I'm afraid) and know how to trace the cable in the walls and under the house, looking for splitters and such that may be faulty or bypassed.