According to Article 50 of the Procedures for Handling Road Traffic Accidents, if the cause of the road traffic accident cannot be ascertained, the traffic administrative department of the public security organ shall issue a letter of confirmation of the road traffic accident, stating the time and place of the road traffic accident, the situation of the parties and the facts ascertained through investigation, and serve them to the parties respectively. Accordingly, if the traffic police do not issue an accident certificate, they can ask the traffic police to issue an accident certificate. After getting the traffic police's accident certificate, you can bring a compensation lawsuit to the people's court according to the relevant evidence you have.