The IOU is a proof that both parties have arrears. If the IOU is lost, other evidence must be found to prove the fact that both parties have arrears.
 You can negotiate with the other party to fill in an IOU. If the negotiation fails, you can also sue, provided that there is other evidence that can prove the debt relationship and is within the limitation of action. Evidence includes audio and video recordings, payment vouchers, witness testimony, etc.
 [Legal Basis]
 According to Article 63 of the Civil Procedure Law, evidence includes:
 （1） Statement of the parties;
 （2） Documentary evidence;
 （3） Physical evidence;
 （4） Audio-visual materials;
 （5） Electronic data;
 （6） Witness testimony;
 （7） Appraisal opinions;
 （8） Record of inspection. Evidence must be verified before it can be used as the basis for ascertaining facts.