Legal Question in Civil Litigation in Pennsylvania

So, they did not sign any promissory note? That is going to be problematic as they can later claim that the money was a gift. Did you pay them via check and write "loan" on the memo line or do you have some other writing indicating that this was a loan?

Never lend money in the future without a promissory note specifying when the money is to be repaid and the interest rate.

How long ago did you make the loan? What I would do is send them a letter and indicate that the funds were loaned and that some time has passed and that they need to repay the debt, either in a lump sum or in installments. If in installments, specify the amount per month and the interest. Get them to acknowledge this in writing.

If they fail to pay, you can sue, but they may try to argue that it is a gift.