Can an employer decide not to let you go after you were told you were being let go? If you were told that if you stayed through most of the transition, you would be eligible for severance and now, after I resigned, they are saying no?

On Feb 16, I was told that my company was doing away with my department. The last day for the Department was April 3, however being the manager, I was not given a final date. My director and I were told that we could seek other employment and would be eligible for a severance package if we stayed through the transition. My director resigned on 2/17 and left 3/9 and was eligible to receive severance. I resigned on 3/14 - I was off that day and emailed my letter of resignation. Simultaneously, HR had called my work phone ( my personal cell must be available for calls) and left a message for me to call. I spoke with HR today and was told that the reason that they were calling me was because they had decided not to let me go. However I had already resigned. In addition, because I was resigning, I would not be eligible for the severance. My last day is 4/3 which is the last day for the department. My question, can they just decide to keep me after I resigned? Do I have a legal case?