I was doing algebra problems when it happened
Politico has an excellent piece of oral history walking through the day from the perspective of White House staff (and occupants):
We May Have to Shoot Down This Plane
Highly recommended.

We May Have to Shoot Down This Plane
Highly recommended.

Then my sister called me and told me to turn the TV on.
I succeeded in calling unemployment on Friday.
Then Jerry Pournelle posted an email from the father of one of the men on that plane.
People were asking him whether he had contacted the news media.
The remainder of the conference was cancelled, and most attendees were stranded, but we drove back to New York (hour north of NYC) at the end of the week. Almost every overpass on the highway had flags draped over the bridges, and we were passed by hundreds of trucks with labels on them indicating that they were full of donations from various States.
Had me weeping all over my keyboard.