How did the loss of the silk road trigger the portuguese exploration?

The Silk Road was a historical route for trade on which the goods from China (silk) and India (spices) were brought to Europe and sold. This route was providing good work and revenue for the merchants, but when it was closed (due to political changes on its route), the people searched for new ways of obtaining goods from India and China - and those new ways were what triggered the exploration - the hope for finding a new way to India and China. 

If the Portuguese could not trade by land, they wanted to find an alternative route to Asia - WATER! Therefore, that's when the Exploration Age began. The Portuguese wanted to discover a water route to India and the West Indies to trade for spices. You will see that explorers like Vasco da Gama and Bartolomeu Dias were famous navigators that attempted to do that.

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