You're thinking of the Tenotichitlan (present day downtown Mexico City) pyramid. When Cortés arrived, Teotihuacán had been abandoned for nearly 1500 years, and was mostly covered with brush. I don't think he ever saw the place. Excavation only started in the late 19th century.
Yeah, I was thinking of that one. Another interesting point is that Cortez had that pyramid torn down and the blocks laid out to form the patio in front of the huge cathedral in Mexico City. It's my understanding that that patio (the Zocalo) is the largest of its kind in the world.
I've visited Mexico City several times (I lived a couple of hours outside Mexico City for about a year), and love the history of that place....