With the fall of Constantinople in 1453 to the Ottoman Turks the Byzantine Empire came to an end. But cracks had been appearing in the empire long before its final capture thanks to a mix of internal economic instability, reduced military capability and an increasing number of external enemies.
Internal instability came in the form of the rise in nobility throughout Asia Minor after 1000 A.D. What had been an area of tax paying free peasants who were given land in return for military service became one controlled by nobles who gradually enserfed these small farm owning peasants reducing not only the empire's tax base but its army as well.
The reduction in the empire's army meant it had to rely more and more on foreign mercenaries, while less taxation resulted in a declining navy forcing it to rely on foreign city states such as Venice to fight its naval battles. In return it offered Venice free trading rights within Constantinople which enabled the Italians to sell goods at a cheaper rate than the Byzantines reducing the empire's income yet further.
Externally the empire faced problems too in the form of the Turks who were invading its eastern frontiers. A heavy defeat at Manzikert in 1071 A.D at the hands of the Seljuk Turks saw the empire lose much of Asia Minor, while in the west it faced a new threat in the form of western powers such as invading Norman forces and the Italian city states which now controlled much of the Byzantine economy. A schism between the eastern Orthodox and the western Roman Catholic churches in 1054 only exacerbated the problems leading to mistrust and animosity. This exploded in 1182 when the Byzantines massacred thousands of Italians living in Constantinople. In 1204 the Fourth Crusade was redirected towards Constantinople on the orders of the Venetians and the city was sacked and captured by the western powers. Recovered by the Byzantines 60 years later the city and the empire was a shadow of its former self and was to last a further two hundred years. From 1300 A.D onwards the Ottoman Turks would push further into Byzantine lands until only the capital Constantinople remained. But in 1453 this too was to fall at the hands of the Turk Mehmet the Conqueror sealing the end of the 1000 year old Byzantine Empire.