Resource Scheduler suddenly stopped starting/stopping EC2 instances on schedule

If your Resource Scheduler has suddenly stopped starting and stopping your EC2 instances as expected, there are several potential causes to investigate:

1. Check the state of your instances when the schedule is supposed to trigger. Resource Scheduler only starts instances that are in a "Stopped" state and only stops instances that are in a "Running" state. If you manually changed the instance state outside the schedule, the scheduler won't override this.

2. Verify your schedule and period settings, ensuring that the weekdays, begin time, end time, and timezone are correctly configured.

3. Check the IAM role settings and access permissions. If your EC2 instance uses encrypted EBS volumes, ensure that the Instance Scheduler has permission to use the associated AWS KMS key.

4. Confirm that the CloudFormation stack for the scheduler is in the same Region as your instances. If not, you may need to create a new stack in the correct Region.

5. For newer versions of Instance Scheduler (v3.1+), verify that the IS-ManagedBy tag is properly applied. Sometimes deleting and recreating the tag can help retrigger registration.

6. Check the scheduler administration logs for any hub registration errors or other issues that might explain why scheduling stopped working.

Since the scheduler was working correctly for a week before the issue occurred, it's possible that something changed in your environment or that there was an API operation failure. Reviewing these configurations should help identify the cause of the problem.
Sources
Troubleshoot the Instance Scheduler on AWS for EC2 instances | AWS re:Post
Stop and start EC2 instances automatically on a schedule using Quick Setup - AWS Systems Manager
Troubleshooting - Instance Scheduler on AWS