If it is possible, perhaps you can try to speak to them in person. Can you make an appointment? I know that you tried to write but for whatever reason, your bosses did not respond. If you do go to see them in person, you should try to be as calm and respectful as possible requesting that you wish to have an explanation for why you were taken off the program. Tell them how much time, effort and energy you placed into the role and that you wish to know why it is they felt that you were no longer a match for that role. Maybe talking to them in person can help ease your mind about what happened.
You also have to deal with the fact that you may never receive an answer to what happened. There also might not be a good answer. Even if you heard what your bosses had to say you may not be satisfied with or like their response. Their answer may even anger you more.
My advice, as difficult as this may be, is that you try turning this negative into a positive. View this unfair firing as an opportunity to find a new and better suited role for you. Your situation reminds me of a documentary I saw recently called Fired! by Annebelle Gurwitch. She was fired by Woody Allen after having been cast into one of his plays or musicals (I cannot remember which one). She made the documentary because she was so devastated by being fired by the great writer, producer and actor Woody Allen. In her movie, Ms. Gurwitch interviews individuals and celebrities who had been fired. Interestingly, among those who participated in the film, many individuals, after being fired subsequently went on to get better and more suitable jobs or roles. In the case of Ms. Gurwitch, being fired by Woody Allen led her to embark on the creation of a movie, and essentially catapulted her into a whole new career. If you get a chance, check out the film. You can read about it at http://annabellegurwitch.com/blog2/books/fired/.
I hope this helps. Take care.