The answer is nothing, provided that everything was properly wired and that the power source generator was grounded. This covers most cases, but certainly not all of them.
With a Y-connected generator, it is easy to ground the center of the generator, and it is common practice to do so. With a delta-connected generator, there is no center point, and consequently such systems often have not grounded neutral (the happens in the US Navy in places).
If in doubt, measure the potential between the two wires before you short them.