If the money was a loan, it has to be repaid as per the terms of the loan--if not, you could sue him for breach of contract for violating the loan's terms.If the money was an investment and there was a written agreement as to what it would be used for, you could sue again for breach of contract. If the money was investment, then even without a written agreement, when you invest in (as opposed to loan money to) a business, you become an owner or partner--possibly a minority one, if another person or persons invested more, but still an owner or partner. The other partners owe you a "fiduciary duty," or duty to deal with you in good faith; if it is violated, you can sue about that violation.If he lied about what he was going to do with the money or what you  would get for it, he committed fraud, and you could sue him for fraud.And/or if he simply took money entrusted to him for a specific purpose and used it for something else or his own benefit, he may have committed "theft by deception."So depending on the exact facts and situation, there are several reasons you could potentially sue.