I've had one in 1997, took me more than a year to recover. I couldn't work, sleep, eat etc. I even wanted to die.
I think it's a combination of mental and physical things that go severely wrong. Bottling up things inside yourself that go 200% against your nature and crossing your limits (e.g. little sleep, no balanced food, doing things you hate to do).
I was ignoring signs, lots of signs for a very long time, I was so stupid. Eventually I ended up being tensed night and day, not being able to sleep for months, having one inflammation after another and after a ski holiday with lots of pain the bomb exploded.
Every night I sat for hours on the couch thinking I felt physical pain inside but I couldn't point where, wishing I had a broken leg or something I could point at. That were probably my nerves in my entire body being damaged.
I started getting anxiety/angst attacks that boiled up through my neck and thinking I would get them triggered them even more, anxiety for anxiety. These attacks could stay for hours, shaking like crazy, devouring the little energy that I had left.
At my peak I had hallucinations during the one or two hours I slept at night, seeing colors, hearing things, it was plain horrible. Looking at something silly like a table or a car could start it, I really thought I was going mental, developing some mental illness. I would never wish my worst enemy that. I had to leave hubby and live at my mom's house for 2 months, she patched me up.
Whenever I'm below my level (stress at work or my kid not sleeping for weeks) I feel that anxiety sneeking up, only now I recognise it, some breathing techniques makes it go away.
It's like there's this damaged place and you have to take care of it very well for the rest of your life.