My life has changed seemingly overnight many times, and in both positive and negative ways.
The most "instant" among life-changing events happened 36 years ago (today's the anniversary, actually - so it's odd I'd run into this question) when a drunk and speeding driver hit the car in which I was riding, and the best friend I've grown up with since childhood, was killed; and I was injured.
We were both weeks short of our twenty-first birthdays. It was as if I'd been going along in life on a train, and the train was suddenly derailed. When it was put back on a track that track took it in a whole other direction than it otherwise would have gone. Maybe it's not accurate to say "when it was put back on a track". Maybe it's more accurate to say "when I had enough time, and figure out how the hell I was going to get that monster of a train on the nearest track with whatever resources I had".
Eight months later, there was more "instant" derailment when my father had a massive heart attack that would soon thereafter result in his death.
Quite a bit of my early- and mid-twenties was spent getting used to, figuring out a new plan for, and generally processing a life that had had so much "derailment" so soon, and within such a short period of time; and a life that was a completely different world than the one I'd once known.
Some of who/what has come into my life as a result of my being in that unplanned-for "world" are people and memories I treasure beyond words, so there's life (and lots and lots of it) after derailment. There's also all kinds of happiness and "OK-ness" and any number of wonderful things one would never trade for anything. There's not, however, looking back on a "track" that runs from point-of-origin to the present stop without a stretch, way back at the beginning of the journey, that's crooked and damaged and marks the end of one ride to one destination and the beginning of a different ride to another one. There are things to be learned from the different view, though, and one of them is one that everyone eventually learns anyway; which is that there is never any returning to the beginning of the journey - whether or not there's a broken stretch of tracks along the way. (It turns out that's not such a bad lesson to learn early on.)