Instance stopped working alone

There are several things that could have caused the instance to stop. One of said possible reasons could be underlying hardware failure causing the instance to need to be shutdown. You can view these types of events on your accounts PHD (https://health.aws.amazon.com/health/home#/account/dashboard/open-issues) and setup alerts to let you know when they occur.

You can also setup alerts/notifications via https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-cloudwatch-createalarm.html on the ec2 instance directly and setup custom alarm actions.

Hi,

Although it is not something that happens frequently, an EC2 instance could be stopped due to a scheduled event, when AWS detects irreparable failure of the underlying host for your instance. Could you checked if that is what happened?

AWS sends an email to the email associated with the AWS account prior to the scheduled event, providing details about the event, including the start and end date, but also sends an AWS Health event, which you can monitor and manage by using Amazon CloudWatch Events. Additionally, you can create an EventBridge rule on a scheduled event, and send a notification via SNS or further processing through an AWS Lambda.