what happened to 911 after the impact, did the whole building just collapse?

The Twin Towers did not just vanish all at once, but both buildings did collapse after the plane impacts and the fires that followed. The South Tower came down first, then the North Tower, and the collapses happened within less than two hours of the attacks.

What happened after impact

After the planes hit, fires burned across multiple floors, damage spread through the structures, and the buildings lost the ability to stand. In the North Tower, no one above the impact zone survived; in the South Tower, 18 people managed to escape from above the impact zone before it collapsed.

Did the whole building collapse?

Yes, the Twin Towers collapsed completely, not just a section. The result was a massive dust cloud and the destruction of both 110-story towers.

Wider context

The attacks killed 2,977 people in total, including 2,606 in New York, and changed airport and building security worldwide. The World Trade Center site was later rebuilt, and One World Trade Center now stands there as part of the memorial and new complex.

TL;DR: after the impact, the fires and structural damage led both Twin Towers to fully collapse, first the South Tower and then the North Tower.