General info only, not legal advice, and not your attorney. A few things worth separating out here. A merchant account closure and a held reserve are governed mostly by your merchant processing agreement, which is a contract, so the first question is always what that contract actually says about termination, reserves, and how long they can hold funds.
Regulation E, which an earlier poster mentioned, generally protects consumers on personal accounts for unauthorized electronic transfers. It usually does not cover a business merchant reserve dispute, so I would not lean on it as your main theory. The stronger angles are typically breach of contract and, depending on your state and facts, possibly conversion of the held funds once the contractual hold period passes.
Whatever you do, get your demand in writing with a clear deadline and a specific dollar figure. That creates a record and tends to move these along faster than phone calls.