You sue them for the money. A loan is a contract, and even an unwritten contract/loan can be  enforced (though written ones are far preferrable: it is much easier to show that they exist and their terms). If the borrower did not repay when the agreement was that he should repay, he is in breach of contract and you can sue him for the money. In the lawsuit, you will have to convince the court not just that you gave him money but that it was a loan (e.g. not a gift) and that the agreement had been that he should have repaid it by now.