Instance reachability failed!

Update from support agent-
The kernel changed and that may be the cause of the instance failing.

The question is how did it spontaneously change? Only I use it and I've only been running some Python scripts. Pretty innocuous stuff.

Hi Ace4plasticoMan,

I noted that the Instance in question was failing Instance Status Checks but has since recovered and is now passing both Status Checks.

As noted here [1], Instance Status Checks monitor the software and network configuration of your individual instance. Amazon EC2 checks the health of the instance by sending an address resolution protocol (ARP) request to the ENI. Unfortunately, due to our privacy and security policy, we do not have access to our Customers' resources therefore I cannot confirm the root cause.

I would recommend reviewing your OS level logs such as /var/log/messages during the same timeframe of the issue for any errors, warnings, etc. If you have any further concerns, please let me know.

Thanks!

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/monitoring-system-instance-status-check.html#types-of-instance-status-checks

Shot self in foot. 100% my fault. I edited fstab days before the reboot and forgot.
Issue was resolved with support case.
One agent thought the kernel had changed but second agent did not agree. Commenting out the bad line in /etc/fstab resolved the issue.