1. Don't get angry, and never do something stupid
2. Send them email clarifying all your rights/claims and list all what they need to do to compensate you (money? ask higher than what you want)
3. if they start negotiating the numbers, start negotiating till you reach a good deal that satisfy you. (negotiate slowly, no equations, and always get advice from friends who have good negotiation skills)
4. if friendly negotiations fail, ask your lawyer to take legal action.
A best practice is to have a lawyer from day one. it's a lot of money but worth it. (always ask your lawyer before saying/doing something. your lawyer's job is to prevent you from doing something stupid).
Also, don't let that stop you from moving forward with your life. you built something once, and you will do better with your next startup.
[Been there, done that]