Oops.. I think I was thinking Mayan.. not Aztec.
You might also be thinking about the Toltec. They died out about three centuries before the rise of the Aztec empire.
Actually it is probably wrong to say that the Toltecs died out. They probably did no such thing. What is more likely is that their civilizaton went into a decline as has happened to civilizations all over the world. Their people went on, but not at their previous level of development.