The fall of the Roman Empire is obviously a huge shock for the west. There are many reactions and calls to launch a crusade to capture the city again, the problem is that the european countries are too busy or simply not in position to launch a crusade and thus all the project to drive back the Ottoman ended being mere wishfull thinkings.

-Frederick III of the Holy Roman Empire announced the holding of an Empire Diet in Regensburg in September 1453 (followed by the Diet of Frankfurt in September 1454) to prepare a crusade. However, it ended with a failure, the promise to send 10,000 cavalrymen and 30,000 infantrymen remained a dead letter. The emperor just didn't have enough authority to raise so many men...

-France is exhausted by the hundred year war and, with Burgundy (which Duke also had a quixotic idea of crusade) are staring stonily at each other; there is also the fear of an English backstab if ever they launch a crusade.

-As for England, the country is weakened by the loss of the HYW and led by the weak minded Henry VI, the war of the two roses began in 1455 preventing any foreign actions.

-Poland and Hungary already tried to repell the Ottoman 10 years earlier (Crusade of Varna), but ultimately failed (the death of the king split appart the 2 kingdoms which were ruled by the same king). Hungary is anyway already at war with the ottoman uinder the command of John Hunyadi (defeated in Kosovo in 1448 but victorious at Belgrade in 1456)

-Finally, the most important reaction, the Pope Nicolas V called for a crusade in September 1453, but no strong king really answer to this call. This is a huge symbol, the Pope (weakened by the great western schism) had lost a huge part of his influence and power on the western sovereigns compared to the one it used to have during the central middle ages.

This time the national interest prevails over the defense of the faith. Louis XI of France (known as the Universal Spider for his cunning diplomatic craftyness) accurately summarize this when he claimed: "You are more bound to the king and his country than you are to the emperor of Greece and other lords of the Levant, and it would be no great honor for you wanting to reconquest Constantinople while letting the king and kingdom destroyed by the English"