The likelyhood of alien civilisations existing is very high, its possible they have been on earth but it will be a long time before that would be general knowledge, even a highly advanced civilisation wouldnt be able to pick up on any of out signals unless they were very close as it would take millions of years to reach any systems that could support life, maybe on the off chance a ship flying by our solar system.
If aliens have visited then they probbaly wont come back for a long time until the planet sorts itself out, i think we would be screwed if any bad aliens visited earth unless we are so far on the fringes it may not be worth it to even travel.