Complex sentence: Cuneiform writing was gradually replaced by the Phoenician alphabet during the Neo-Assyrian Empire, and by the 2nd century AD, the script had become extinct, all knowledge of how to read it was forgotten until it began to be deciphered in the 19th century.
Split into simpler sentences: Cuneiform writing was gradually replaced by the Phoenician alphabet during the Neo-Assyrian Empire. By the 2nd century AD, the script had become extinct, and all knowledge of how to read it was lost until it began to be deciphered in the 19th century.