Citi Bank fraud department blocked my account on a cleared check that they believe it was not issued by the issuer how long can they legally block my account or my funds
This is a customer service issue, rather than a legal issue.
You might review your bank's terms set out in their disclosures, and you might also visit your branch in person to discuss this with a branch manager.
Note the coronavirus may have caused disruption in your bank's staffing and hours, so services may be even less efficient than usual.
