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I am sorry to hear about this situation. Someone in your situation may wish to sue the person that you accidentally paid the money. Really, that is the only way to get it back unless they give it back voluntarily.
To sue in a state court, one needs to have a "cause of action." There are numerous causes of action, such as "breach of contract," "negligence," "fraud," "unjust enrichment," etc., as well as causes of action rooted in statutory law. Every state has their own although they are very similar to each other in every state because they all stem from the same common law. A pleading in Court needs at least one cause of action, although it is not unusual to have more than one.
Someone in your situation may wish to threaten (or sue) for money had and received. The elements of such action are: a person has received money of the other that in equity and good conscious he should not be permitted to keep; demand for repayment has been made; and the demand was refused. See Fain v. Neal, supra at 497-499, 103 S.E.2d 437. This applies here arguably as the person received money that in good conscience they should return (and should not have spent).
Often, the threat of filing a lawsuit is enough to have this matter resolved without needing to file suit.
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