I was at work, I happened to be out late the night before in NYC and at one point I was looking up at the towers themselves sometime around midnight to 1AM on September 11th thinking to myself I should come into NYC more often. (I lived in Queens, worked in Westchester, and generally avoided Manhattan proper.) I got home like 3AM, found it hard to sleep, was still young enough (22?) I didn't need to sleep and decided to go into work extra early.
My mother called me at work to tell me a plane hit the tower. I had to hang up with her to find a television in the office. By the time I got to the TV I saw what turned out to be the 2nd plane hitting live. I was confused wondering how did they have a camera to catch the first plane hit and why was there already smoke? Then we all realized what was happening. One of the guys I worked with had his wife who worked in the towers so he went to call her. (She lived, thankfully.)
I had to cross a bridge to get home so I never went home that night, the company had a few empty apartments nearby and me and another worker got the keys to one for the night.
Those who weren't alive or weren't old enough may never understand how much the world changed that day. Even if there was another attack of the same size, it probably wouldn't be the same, because now we almost expect one.
A good friend lost their father that day in the towers. He was the only one from his company to die, they'll never know why or exactly where he was, I assume he may have stayed behind to help others out because he was that kind of person.
It both pains and enrages me when people, especially those who were in diapers when it happened, spout bullcrap about it being an inside job, or planes didn't hit the towers, or they were wired with explosives, or the Jews didn't go to work that day (My friend's father who died happened to be Jewish.) I don't care than 200 engineers signed a report saying the official story isn't true, there are millions of engineers in this country who did not sign challenging the report. If you lived it you know what happened. We let our guard down and they got us.