Depends on what kind of Atlantis. If is was a large atlantic island with a super advanced technology in Plato's time, then who knows.
If it was just a large, inhabited landmass in the eastern Atlantic with a typical bronze-age, but maritime, civilization, then it would have had many effects on the rise of European and Mediterranean civilization, and quite possibly helped create contact between old and new worlds at least 2000 years before Columbus. THis would have had an interesting effect on the rise of early Mesoamerican civliization, which was significantly less advanced than the mediterranean area, but not disastrously so. Probably a merged civilization would develop in the Americas, combining long term Mexican cultural elements (corn, beans, squash agriculture, blood religions, kingdoms) with old world technological and cultural influences (bronze metalwork, maritime technology, draft animals, more sophisticated scripts)