If it is connected to a water main then it is very unlikely that you have an air lock as the pressure will just force the water through. When you emptied the cylinder did it fill up again when you opened a hot tap and did water eventually come out of the tap after expelling the air?
If not it would seem that the tap is not opening or there is a blockage between the cylinder HW output and the tap..
PS The above scenario is what happens in a domestic premises when you refilll a cylinder.