The Byzantine navy had already been dissolved in the 1320s; Venice and Genoa ruled the waves.
The crown jewels were pawned off in 1343, never to be redeemed.
Byzantium had been wracked by civil war for decades; and the civil wars were being fought on behalf of the factions by Serbs and Turks.
Gallipoli was occupied by the Ottomans in 1354, establishing an Ottoman presence in Europe; the Byzantines re-took it in 1366 (through the Savoyard crusade, the last time the West did anything to contain the Ottomans for a long time); and the Byzantines surrendered it back in 1376, to repay debts incurred in yet another civil war. Once the Ottomans were in Europe, it was all over.
The Byzantine emperor John V (1354–1391), humiliatingly, was reduced to begging for help in Western courts.
The Serbs were defeated in 1371, and the Ottoman Emirate was the only regional power that mattered any more.
Byzantium was a vassal state of the Ottomans by 1372, and John V was obligated to take part in the emir’s military campaigns.