Christian Europe had conducted overland trade with India and China for products like silk and spices. But the Mogul Empire, which had allowed this trade to pass through lands under its control, started to fragment. The emerging Muslim states in the area started to prevent free movement of trade to Europe. The Turks captured Constantinople in 1453. The price of goods from the Far East rocketed. Portugal then tried to find a sea route eastwards to the Indies. Columbus had a different solution, his proposal was to travel west across the Atlantic to the Indies.
It is sometimes claimed that Columbus had difficulty obtaining finance for his plan because Europeans believed that the earth was flat, a claim mooted in Washington Irving's 1828 novel, The Life and Voyages of Christopher Columbus. In truth, it was not the shape of the earth that was an issue, but the circumference and hence the distances involved.
Most academics, in Columbus day, accepted Ptolemy's hypothesis that the world's landmass (for Europeans of the time this meant the known world of Eurasia and Africa) occupied 180 degrees of the terrestrial sphere, leaving 180 degrees of water to be crossed to get to Asia westwards. Columbus believed that 1° represented a shorter distance on the earth's surface than was commonly held. He calculated the circumference of the Earth as 25,255 km at most. The true circumference of the Earth is actually about 40,000 km. No ship in the 15th century could carry enough food for voyages of the real distance. Most European sailors concluded that undertaking a westward voyage from Europe to Asia would be impossible as crews would die of starvation or thirst before reaching Asia. LINK