I was ignorant of the attacks (bear with me for a minute).
On September 11, 2001, I had been in Japan to study abroad for about 3 weeks, and was just getting used to my host family. The attacks occurred between ~10 PM and midnight (local to me, anyway), and I didn’t have an internet connection at my host family’s house yet.
The first I knew anything had happened was the next morning, when I saw that a friend had texted me, telling me to turn on the TV. He didn’t say what had happened, though.
I got to campus, and my friends all asked if I was all right; I said yes, and didn’t think twice about it, until my parents called and told me what had happened.
My family and friends were safe; none of us lived near the attacks. The closest anyone I know came to being involved (and I didn’t find out about this until afterwards) was another of my college friends (who was en route to study abroad in Scotland); he flew out of New Jersey a few scant hours before the attacks (IIRC, anyway).
The Japanese were very sympathetic; I read online about certain other foreigners claiming that it was nothing less than the US deserved, but the Japanese (at least those I talked to) didn’t.
I essentially got two culture shocks for the price of one; the first was when I arrived in Japan and had to get used to how it was different from the US.
The second was when I returned home to the post 9-11 US (in June of 2002), and had to get used to how things had drastically changed since I left. The world changed that day; it just took a bit longer for me to experience it.
My condolences to everyone who had loved ones taken from them by murderous terrorists in the attacks on the WTC and the Pentagon, and the loved ones of the passengers of Flight 93, who fought back, saving who knows how many lives at the cost of their own.