This sounds like a pretty good retaliation case, where you were punished for refusing to do something fraudulent.
See an employment litigator to disclose and discuss exactly what you were asked to do, and how your employment conditions changed as a result of your refusal. Prepare a chronological log, with names, date, witnesses, etc. of everything you experienced and witnessed from te time you were hired til the time you were fired.
Disclaimer: I'm only licensed in CA. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
This questions raises a lot of other questions. Depending on the type of fraud involved, you may be protected by whistleblower laws. I would definitely consult an attorney to discuss the details. Many employment laws involve timelines, so you should consult an attorney as soon as possible.