in my family there has been a simple question being raised for as many years i can remember but there has never been a satisfactory answer yet. "if early muslims had all the science, mathematics, geography, philosophy and all fields of science 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?" Then years…
* Much of the "Golden Age of Islam" is actually attributable to cultures which were originally not Islamic at all eg: Persia, The Levant, Mediterranean, Central Asia, Indian subcontinent. The decline started with the "Islamization" of these cultures by the Arabs who were far more "primitive". This is the worst of brawn over brain.
* The rise of "fundamental" Islam; most notably the Ash'ari within Sunni sect (al-Ghazali was just one of the most well known of this group) to the detriment of all the other "forward looking" sects in Islam.
* The Mongol invasion which wiped out a unified Islamic empire leading to more internecine strife within Islam.
* The Christian Crusades.
* The general ossification and fossilization of beliefs in Islamic cultures causing them to turn away from the "Modern Scientific Enlightenment" movement which started in Europe.
Note that similar decline of cultures have happened before due to similar (but not same) reasons; most notably with Ancient China and India though not to the same degree.