Which time?

Julius Cæsar's civil war saw the library burn in 48 BCE during the Siege of Alexandria (Plutarch, Life of Cæsar, 49.6)

Aurelian attacked Alexandria in the late 3rd century CE to suppress a revolt by Zenobia, and the area around the library was damaged during the fighting (Ammianus Marcellinus, 22.16)

Patriarch Pope Theophilius of Alexandria destroyed the Serapeum in 391, after Theodosius I declared paganism illegal (Socrates of Constantinople, Historia Ecclesiastica, 5.16)

Finally, 12th century and later sources indicate that the library was damaged or destroyed again during the Islamic conquest in 642. The earliest account is from Abd'l Latif of Baghdad, in the late 12th century, while the latest is from Al-Maqrizi in the late 14th/early 15th century.