I'm aware of Fall of Constantinople in 1453 being a major turning point because it blocked the trade route into Asia with the Ottoman Empire taking over. This caused serious economic pressure for Western Europe and may have played a role in the persecution of the Jewish population at the time. Obviously in countries like Spain, religious dogmatism played a big role being under Catholic rule.

The most famous expeditions are Vasco da Gama's travels across the African continent into India and of course Christopher Columbus sailing into North America. The latter if which I believe spent years campaigning to the crown of Ferdinand and Isabella of Spain to fund his expedition. It's fascinating how people became famous by perfect timing.

So what other incentives led to the days of early exploration in the late 15th century?