My ec2 instance reboot itself

It would be worthwhile setting up CloudWatch agent to collect more detailed system logs https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/Install-CloudWatch-Agent.html

This will show you more detailed system resource usage, and while this may not be much use in tracking down why previous reboots happened, it could provide data which may be useful if it happens again.

You may need to investigate more at the guest operating system level, troubleshooting steps here will very much be dependent on the OS running on the EC2 instance. If it's Red Hat Enterprise Linux then this is a good place to start https://access.redhat.com/articles/2642741