What if the ex-boyfriend doesn't pay back the money? Ask for advice?

Did you write an iou when you lent it to your ex-boyfriend? If not, you can communicate first, make up the iou and give the other party a period of time. If the other party still says that he has no money after a period of time, you can claim it through legal proceedings, write a complaint and sort out the evidence. If you don't know what information to sort out, you can ask for help. If the court in the jurisdiction receives it, it will issue a summons to the other party, and generally it will receive the court.