When did Egyptians stop being able to read hieroglyphs? And why? 

Egyptian Hieroglyphs are one of the most well known writing systems from the ancient world, and they were employed for over three thousand years, from the Old Kingdom period through the Roman era. Now, though, they are no longer used as a script. So when, and why, did they die out?
SOURCES:
The Final Pagan Generation, Watts
The Rise of Western Christendom, Brown
The Oxford Handbook of Roman Egypt, Riggs