The fall of Constantinople did send shock-waves across Europe, particularly in Italy. The first places to hear of the fall were Venetian colonies on Crete, who quickly sent a message to the Venetian Senate. Over the course of the siege, the Senate had been debating whether or not to send military aid to Constantinople and ultimately decided to do nothing in fear that their rival Genoa would try to take advantage of the situation. When word finally reached the Senate which was in secession at the time, that secession was suspended. Word quickly got out about the Turks' brutality and how Italy seems to be next to fall to the Ottomans, who did stage an invasion of Italy in 1481 only to turn back on Mehmet's death. In sum, I would say that the fall of Constantinople and the almost immediate conversion of the Hagia Sophia struck fear into Europeans that the Turks were unstoppable and were going to sniff out independent Christian rule as they quickly consolidated the Eastern Mediterranean under their rule. While calls for Christian unity were issued out and Holy Leagues did form from time to time to combat the Ottomans, the truth of the matter was that the Christian unity of the Middle Ages was gone and few countries outside of Hungary ( the European state closest to the Ottomans) had any real interest in Crusades.

Side-question: I just looked it up and it seems that it's the same as "Aya Sophia" that I remember. Is there some history on the spelling variation?

Couldn't the Venetians have saved Constantinople? I think there is a quote somewhere stating, "We are Venetians first, Christians second" or something along those lines.