I was airborne on a flight from Kansas City to O'Hare, as a UAL 727 flight engineer. We never got the word while airborne to secure the cockpit. When we got to O'Hare, we landed, and realized something was up: nothing was moving on the ground. Weird. We were sent to a piece of tarmac and told to park.
Over the next hour or so, we got SA on the situation from various sources. At first, I thought it was a massive power failure. Then, a cryptic message came down through the ACARS (onboard data link) that made me think a nuke went off.
I remember when the second tower came down, that a pax told me her brother worked in that tower.
When we finally got inside, I stayed in ops a while. When I ventured out, I was one of the only people in the entire airport. You just can't imagine walking through O'Hare and not seeing anyone, except a few security folks.
I spent 4 or 5 days there, and finally got onto the first airplane that went from O'Hare to San Antonio (my home at the time).