That depends on your definition of ‘lost or dead.’ While conversational Hebrew gradually died out between 300BC and 400AD, and remained essentially unused as a spoken language for the next 1500 years, it remained continuously used as a liturgical and literary language. Spoken in ancient times, Hebrew, a member of the Canaanite branch of the Semitic language family, was supplanted as the Jewish vernacular by the western dialect of Aramaic beginning in the third century BC, after the Babylonian Captivity. The common Aramaic also died out in 1100AD, replaced by Arabic.
The revival of Hebrew as a spoken language, lead by Eliezer Ben-Yehuda in the 19th and 20th centuries, became the foundation of the official language of Israel. Modern Hebrew used Biblical Hebrew roots, Mishnaic (1st century AD spoken) spelling, and Sephardic (15th century AD Jews in Spain and Portugal) pronunciation, and included many sayings and phrase translations inherited from Yiddish (central European mixture).