Postgresql often shuts down in ec2 instance?

I created a free ec2 instance in AWS. I installed PostgreSQL there. I deployed an application. If I do not open my web application for two days or three days. Postgresql shuts down. Why I do not know. It should keep being alive. I do not want to restart again and again. please give me a suggestion to finish it off.

error:
timestamp: "2020-03-11T08:46:29.895+0000"
status: 500
error: "Internal Server Error"
message: "Could not open JPA EntityManager for transaction; nested exception is org.hibernate.exception.JDBCConnectionException: Unable to acquire JDBC Connection"