EC2 instance - Instance reachability check failed issue after reboot

The "Instance reachability check failed" issue you're experiencing after rebooting your EC2 instance can be caused by various factors. Based on the system log you provided, it appears that the Amazon SSM Agent service is failing to start, which could be contributing to the reachability issue.

Here are some steps you can take to troubleshoot and potentially resolve the problem:

1. Check system resources: Ensure that your instance has sufficient CPU, memory, and storage capacity. High resource utilization can cause status check failures.

2. Investigate network connectivity: Verify your instance's network configuration and security group settings to ensure proper connectivity.

3. Examine system logs: Look for additional error messages or unusual activity in the system logs that might indicate the root cause of the problem.

4. Troubleshoot the SSM Agent: Since the log shows a failure to start the SSM Agent service, try manually starting or reinstalling the SSM Agent on your instance.

5. Check for AWS system issues: Monitor the AWS Service Health Dashboard for any reported issues in your region that might be affecting EC2 instances.

6. Consider instance type compatibility: Ensure that your chosen instance type is compatible with your operating system and workload requirements.

7. For EBS-backed instances: If the issue persists, you may want to stop and start the instance (not just reboot), which typically migrates it to a new host.

8. Use EC2 Rescue tools: Consider using EC2Rescue for Linux or Windows (depending on your OS) to diagnose and troubleshoot the instance.

9. Check the EC2 Serial Console: Access the serial console output for more detailed boot and system information that might not be visible in regular system logs.

If none of these steps resolve the issue, you may need to contact AWS Support for further assistance, especially if the problem persists even after launching a new instance from an AMI.
Sources
AWS EC2 status check failed | AWS re:Post
Instance reachability check failed, can't access the instance | AWS re:Post
Troubleshoot issues with Amazon EC2 instances - Amazon Elastic Compute Cloud