help!

I quitted my job last september because of pregnancy, however, my supervisor terminated my H1B in advance leading to me out of status suddenly, and they told me that they will help me to move back to the original date only if I can do volunteer in the lab (they said they want to punish me because of my leaving), it was finally solved with the intervention of human resource. Now, i start to seek a new job, but the problem is: everytime when the hiring person require my reference letter from that supervisor, i told them i can't provide because there was an unhappy thing happened between us in the end, then i will not hear any reply from them anymore. i'm so sad, that was my first job in US after graduation, now it is so hard for me to find a new job because of that. Is there any helpful advice for my job searching? how should i do if the hiring person ask me again? Thanks very very much.