Many conspiracy theorists claim that the government is keeping things from us because civilians cannot accept an unthinkable truth; that perhaps we not only are visited by aliens, but they might be our creators.
The implications for such an event include religious, sociological, financial, military, scientific and individual.
In my own scientific-minded way, I tend to see the possibility more like the scenario in "The Mist." The government or scientists or a combination of both find a way to break through into a parallel world/other dimension, and out spills the secret to the universe, that we are nothing more than a minuscule bubble in a tub overfilled with soap.
The other possibility might come in a scenario like the one being played out right now with talk of Bigfoot DNA revealing a cross mixing of humans with something that supposedly is not that compatible, raising the question of hybridization by alien masters. By any stretch of the imagination, should there be truth to that, then perhaps we are being "allowed" now to probe our DNA and learn about our "papa" in preparation for our encounter with our creator(s).
Whether the aliens come from a parallel world or another planet, we would be at their whim since they know where to find us, but we do not have the capability to find them. Utilizing logic, we'd have to understand that obviously they do not want to hurt us because they have not taken over so far and humans have been here for many thousands of years. We would be more likely a pet experiment or a test lab for new species. Kind of like the cast-off island of defective toys like in Rudolph the Red-Nosed Reindeer. In fact, perhaps we are more like the feral children of alien breeding experiment left to gestate and occasionally observed to see progression but without any helpful instruction from our "parents."
How mankind handles such an encounter would depend largely on the government and the media. It would take a lot of reassurances and guidance to help the population through such a disruption and maybe, just maybe, we would be better for knowing them, losing a bit of our arrogance that we are the only intelligent beings coming to conclusions about how the world works. An older mentor might just be the ticket to help us advance technologically as well as emotionally and spiritually.