I think it is almost certain that a vast amount of human knowledge was lost. The policy of the city of Alexandria was to copy every book that went through it, and give the copy back. As a result, they ended up with a spectacularly large collection of original materials.

The amount of ancient literature that is no longer available is vast. History, literature, you name it, so much has been lost. The odds are very good that at least a significant portion of that were in the Alexandrian library.