I was driving to work. About 10 min away. All I kept thinking was "is it our building? Is it where our office is?" It was confusing because we always called them WTC1/WTC2 vs North & South towers which was how the news was reporting.
There were very few tv's in the building at that time. No internet (overloaded) so we were all crowded around the tv's in the gym. When our building fell (wtc1/north) we were all crying. We didn't know if they had made it out. I went to the bathroom and threw up.
The next couple of days were crazy. We went to Frys and bought computers, monitors and printers and then rented a u-haul and a couple of guys drove the stuff up to NJ to set the

employees up in our port office there. Our primary hardware vendor hired a truck to drive to Atlanta to pick up more computers and servers in Atlanta and get them to NJ by the time our network guys got there. (Remember all flights were grounded and we knew everything would be back logged). We had the employees back up within a week.
All the executives from NY were in Houston for board meetings.