Do I have a case against my employer because I was demoted due to challenging illegal practices?

I was demoted at my job I believe for 2 reasons: 1 was that I was challenging illegal practices occuring; 2: I went on a medical leave. For the past 6 months my immediate supervisor and I have had several debated about illegal practices occuring which he eluded to politics. Prior to me going on medical, I was informed that I would be promoted. Once I returned from medical I was no longer considered for the position until I inquired. There were two positions: 1 candidate was just promoted and there was no interview and she had less credentials than I. I was finally interviewed once I forced this issue and I was never told that I did not receive the job. Someone just showed up to work. Do I have a case?