Context: I am a nineteen-year-old sophomore at a small college who is studying science. The summer of my freshman year, I did a few months of research for a research mentor who works as a professor at the college. It was a great job and I'm happy that I did it. This year I work for a different professor and so this mentor is no longer my mentor or employer. Everything I have been doing for him now is only to present my research to the department and others.
The problem: This year, there was a seminar in the department to present the research of students who worked over the summer. It was a prelude to the larger conference later this year that I am planning to go to for certain. Unfortunately through a series of events that was beyond my control and which isn't relevant to this question, I missed going to this first seminar to present my research.
Obviously, I felt horrible about not living up to my promise to my mentor to go, so I emailed my mentor. I apologized, told him honestly why I had missed the event, explained that I was happy to make it up to him if there was another seminar or event I could attend instead, etc. etc.
His email in reply was... unpleasant. He said that I had broken my research contract, threatened to never write me a letter of recommendation again, threatened to also make sure no one else in the department wrote me a letter again ("did you think about that?"), said that no one would believe me, said I was making excuses, and so forth. He ended by asserting that I obviously don't care about any of this and that I could "believe whatever I wanted." It felt very vitriolic to me and I remember just sitting there in a state of shock reading this viciously worded email from a former mentor.
Now I don't know what to do. I don't recall that this seminar was explicitly in my research contract, but if it was, then I don't want him to accuse me of breaking my contract, which I know could have rather frightening legal consequences. I'm a young adult who's struggling financially (read: paying college tuition) and can't afford to hire a lawyer. The summer research was my first real full-time job, and he was my first real "boss." I have no idea how to resolve this situation because I have no experience with any of this. Have I made a more serious mistake than I thought? Was his response rational to the situation or was it as hostile and unreasonable as it seemed to me at the time? How do I resolve this situation in a way that doesn't do severe damage to my career?