There was never really "a burning" that destroyed it in one go. Over many years, there were numerous factors that caused many of the works to be lost. Some were lost to smaller fires, some were stolen, some just disintegrated due to age. If no one bothered to copy them by hand to have more than one copy, that was that.

The library shrank over a long period due to the change in patronage, from rulers friendly to academia to some that had other priorities. The writings were taken to other places that were more interested in them. There was indeed a fire at some point but it only burned part of it and anyway by that time the collection was a fraction of what it used to be.