I agree with many of the sibling replies but I would like to address different point.
> "including hundreds of scientists who laid down the foundations of modern science with their works, what happened 800 years ago that all that science just vanished?"
That science didn't "just vanish". If it did, how would it have "laid down the foundations of modern science"? Yes, definitely the "Siege of Baghdad" [1] had a major impact but lot (no claims about how much) of knowledge later influenced the western "awakening". [2]
The awesome thing is: non of this is new. Humanity always worked that way. For example wikipedia also have articles on the Greek and Indian influences to that "enlightenment" era in the islamic world. [3][4]
DISCLAIMER: I don't actually know much about the history in detail, I am sure there is much more that can be said, due to some past cursory knowledge I was just able to look things up.