However I am sure it wasn't 9/11 that did it, because I wasn't really on the internet then, and HN didn't exist.
I can't speak for other countries, but both of these events were like a one-two punch to the gut socially, politically, and economically here in America, and we're still reeling.
In regards to tech and the Internet in particular, I think the Snowden revelations and the PRISM stuff was a key moment, in terms of changing the perception of the Internet. After broad public awareness of ubiquitous online surveillance became a thing, the Internet lost some of it luster and became more threatening and less welcoming.
More recently, the Cambridge Analytica stuff, along with all the major data breaches over the past few years, probably contributed to that sensation as well.
I miss the trajectory. Many things were exciting in the '90s, but many things didn't turn out so well.
This is pretty much the same way I feel. I'm not sure exactly when things changed, but there is definitely a sense that we went off the rails somewhere along the way. Some time ago there was excitement, optimism, and a sense of boundless opportunity, associated with the Internet and tech in general. Now... well... there's still some of that, but a lot of it has been replaced by negativity, fear, pessimism, doubt, and suchlike. The overall zeitgeist is definitely less pleasant these days, IMO.