I was at home, drinking coffee watching the news. Had an interview for a new job scheduled. News came on about an airplane hitting the first tower. I remember thinking that I couldn't get stuck watching that or I'd be late.
Then the other plane hit the other tower, I saw that live. I put a VHS tape in the VCR after the second hit without thinking, maybe I was in shock and I didn't know what was happening and thought I may need to re-watch emergency broadcast system instructions (that may sound dramatic, but at the time nobody knew what was happening other than this was serious). I have about 4 hours of the news coverage recorded, though it's just up in the attic and I've never watched it.
I was just in shock after that second hit. I flipped between news channels.. I landed on a channel doing a live report from the Pentagon. This was a "get the Pentagon's reaction" thing. And then the reporter looked up and said he just heard something. So now one plane, two planes and then the Pentagon hit and wow that's total SHTF. At some point the buildings went down.. the whole cascade of events was a nightmare unfolding on TV. The worst was people didn't know how bad it would get, if this was going to happen in cities all over the nation. Could a nuke be involved too.. bad stuff.
Luckily, that Pentagon hit was the last of it but at the time it felt like the world had gone crazy and the country was under coordinated attack. I fully expected more attacks.
I never made the interview by the way, but nobody noticed and I rescheduled.
I was also taking some college classes at the time. In the following days and weeks, I can remember sitting in an algebra class (of all things) and thinking how pointless it was. Major world events going on, how do you concentrate on algebra. That's what was odd, these major events on a national scale yet locally everything is the same. Sort of a cognitive disconnect there. I do remember how odd it was to not see planes in the sky. All radio stations stopped playing music, went to news. The top 40 station played Amazing Grace.
Anyhow 9/11 really effected me, to be honest more than what's healthy. I was younger then, it put me in a funk for a while.. totally shattered my assumptions about the world. This sort of thing was supposed to happen in fiction not reality. I'd just read Tom Clancy's novel where he wrote pretty much the exact scenario -- at the time, I thought Clancy was stretching believability. And then it happens for real.

9/11 was a loss of innocence.