If you can provide any evidence at all of the agreement that you had, for example emails, text messages or even friends or family who knew about the loan who would be willing to testify or as mentioned above, regular payments being made from your friend in repayment of the loan, which evidence the existence of a loan on the balance of probabilities it will be up to the borrower to justify why such payments were being made if no loan existed.
I would start by sending a letter of demand, requesting resumption of payments by specific date failing which you will commence legal proceedings.