I had a long period of time where I couldn't get motivated to do anything after a business partner screwed me out of millions of dollars from an acquisition. I would try and try, but I just couldn't get started on things, or I would start and never finish. I later learned that after this incident, I was suffering from "learned helplessness" [1]. This is a phenomenon where after experiencing a trauma that you were powerless to stop, the brain generalizes that powerlessness and applies it even to situations where you do have control. Essentially, you no longer believe that your efforts can yield positive outcomes, and you stop trying.

Learned helplessness is believed to be a major driver in depression and obviously is a source of issues with motivation. It's a really interesting and treatable condition. If anyone reading this believes this may be an issue for you, you should look into a therapist that is experienced with CBT, as this can be an effective treatment for learned helplessness.

[1] https://en.wikipedia.org/wiki/Learned_helplessness