I've experienced what would probably have been called a "nervous breakdown." For me it was a part of severe depression, combined with external life stressors. I was in my fourth year of university, and I'd had a pretty rough year. The year before had been great--year abroad, lots of travel, first love. Then, right at the height of that, I got really ill (I have a chronic illness, but it had never been that big a deal). I spent the next few months really struggling with my (physical) health, but wanted to finish my degree. That fall was just awful. I was disliking the university and location before the year abroad, and coming back to it afterwards was even worse. That first love was still ongoing, but it was long-distance, and also a stupid "not friendship-not relationship" thing. My dad was four hours drive away; my mom was across the country. My two best friends there were a couple, which inevitably left me out. My health was still iffy. I was trying to figure out what to do next--I'd always planned on grad school in the UK, but suddenly my health was a factor. I don't have good self-esteem at the best of times. And I have a strong family history of depression. I plummeted.
The actual "nervous breakdown bit" happened in November. My health was getting worse but I couldn't deal with facing it and didn't admit it. I'd just started anti-depressants, but they take a few weeks to kick in. At that point the depression was so bad it could take me an hour to work myself up to getting dressed. My "not-quite-boyfriend" had actually been amazing, incredibly kind and supportive. So when he told me flat-out, "there's no chance of a relationship while there's distance, and p.s. there's someone else".... it pushed me over the edge. I spent the weekend crying and crying. It wasn't just the heartbreak, though; it was the feeling my life had well and truly gone to pieces. I had an essay due on maybe the Wednesday. I've always been a terrific student, As and A pluses and firsts in the UK. With everything else going on in my life I was already incredibly behind on this paper, but I had enough to get something out of.
I could not do it. I spent hours and hours unable to write a sentence. In history, my favourite subject, on a topic that interested me. It was really frightening. I wasn't thinking very rationally either, so when I couldn't do it the night before I figured, well, class isn't until 2, if I wake up at 8 that's at least 5 hours to work on it, papers don't usually take me more than 5 hours, I can do that. Of course the morning was even worse. At one point I actually lay on the floor crying. Sometime before class I wrote some message to my professor apologizing, flat-out saying I was struggling with issues of physical and mental health, which I of course wrote through nearly hysterical tears. I had (other) professors seriously encouraging me to apply to Oxford, and I couldn't write a paragraph of my paper.
I actually don't remember much of that week. I don't think I went to any of that particular class, and I doubt I managed many other ones (before that fall, I'd missed maybe one class in three years of university). I'm pretty sure I barely ate. Luckily my dad was "only" four hours away, and I went home for the weekend, and anyway the breakdown had impacted my physical health pretty dramatically too and I ended up in the hospital for a couple weeks for that, which got me leeway on the school stuff. I hope I would have had the same understanding for the mental issues, but it's hard to know, even these days. I've written papers while very ill before, though, and got top marks. That time I'd just fallen apart.
So, well, in brief-- for me, I'd say it was the combination of exterior events, emotional vulnerability, and untreated depression. I was on the edge for a couple months before that, certainly. I've since experienced worse exterior events, with lots of health issues, unhappiness, and lack of control in my life, and certainly struggled, but never fallen apart like that again. The ultimate break-up with that fellow was bad (it went on two more years...agh), but my state was nothing like it was during this breakdown. I've been on anti-depressants all that time, which I suppose is treatment, though they weren't prescribed for that specifically.
There's definitely different angles to the "nervous breakdown" label, but, anyway, that's my depression-based one!