Honestly, the Fourth Crusade in 1204 was more of a "real" death of the Byzantine Empire than the conquest of Constantinople in 1453. Although the largest remnant of the Byzantine Empire was able to reconquest Constantinople in 1261, the city's population never recovered (it went from ~400k in 1204 to ~50k in 1453). The 14th century saw it riven with a series of civil wars, which the Ottomans used to expand their foothold into the remnants of the Ottoman Empire. By 1453, Constantinople was unable to really defend itself without garrison from the major European states like Hungary and Venice, and Mehmet II was able to conquer the city before those states could get their forces sent out.