There was a major effort by surrounding European powers to curb Ottoman expansion that was sanctioned by the Pope called the Crusade of Varna in 1443-44. This was a major military campaign against the Ottomans that included forces from Poland, Bohemia, Hungary, Lithuania, and the Teutonic Order. This crusade wasn't a direct response to Byzantine pleas for aid from the West but instead was a response to border clashes between Hungary and the Ottomans, but the Crusade's goal aligned much with the Byzantines to curb Ottoman dominance in the Balkans and make the Ottomans relinquish their strangle over Constantinople and the surrounding waters.

To say that the crusade's climax at the Battle of Varna was a colossal disaster for the crusaders is an understatement. The crusading army broke in retreat and were annihilated after learning that the King of Poland, Władysław III, was killed on the field. The crusaders themselves were pushed back to defending their own borders against Murad's and later Mehmed's forces.

To put it short, the surrounding European kingdoms attempted to curb Ottoman dominance of the Balkans, but after a defeat so paramount at Varna, no kingdom was in any condition to mount an offensive against the Ottomans, and were barely able to defend their own borders when attacked.

The Ottoman victory at Varna completely opened the way for Mehmed II to strike at Constantinople without any serious foreign intervention, with the only nations answering the Byzantines' call for help was a few thousand Genoese soldiers and some ships. Venice also attempted to send a fleet to Constantinople, but by the time they reached the city it already fell a few days earlier.

If you want some further reading into the failed Crusade, here's a few good reads:

Halecki, Oscar, The Crusade of Varna. New York, 1943

Kenneth Meyer Setton. The Papacy and the Levant, 1204–1571: The fifteenth century. American Philosophical Society, 1976. P. 89–90