The Library of Alexandria was burned down or alleged to have been burned or destroyed several times.
Its even questionable whether it still existed as such in the Christian era.
Several historians told varying accounts of a Arab army led by Amr ibn al 'Aas sacking the city in 642 after the Byzantine army was defeated at the Battle of Heliopolis, and that the commander asked the caliph Umar what to do with the library. He gave the famous answer: "They will either contradict the Koran, in which case they are heresy, or they will agree with it, so they are superfluous."
Apparently the holding in the library at the time were still quite substantial. It should be noted that the (Christian) Byzantine Empire had controlled Alexandria for centuries prior to the conquest by the Arabs.