For me[0] it just happened when I realized that no matter how hard I worked the company I worked for where going to go bankrupt anyway.
I started crying anytime I was alone. Sounds that reminded me about problems at work (e.g. a loose drive belt) would instantly give me pain in my stomach. It took months before I could work in customer facing work again. I was lucky, worked at a farm and completed military (draft) in between and little by little things got better.
I sometimes feel really exhausted and unmotivated, going to work only because it is a duty (to work and my family) but I never burned out again.
[0]: This was before burnouts was a thing so I never got a diagnosis and I only realized later that I hat hit the wall hard.