In re: Citigroup Inc Securities Litigation, U.S. District Court, Southern District of New York, No. 07-09901 is the case, settled in 2013.
If you participated, your account would reflect your piece of the $590 million settlement.
If you did not, I suspect the statute of limitations has likely passed for you to pursue your own litigation. I can't say for sure, but it is likely the settlement proposal you were sent also gave a specific deadline for claims to be filed if you did not want to participate in the group litigation. That time has likely passed.
Sorry I couldn't offer a way to 'open the door'. Citibank lost a huge amount of its value because of its role in the fraud, and never recovered. The bank paid more to the government in penalties, and to Fannie and Freddie, than to its own shareholders.
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