The instance is running, but the console is set to a different region
Each EC2 instance is launched in a specific region, and you can view only one region at a time in the console. Be sure that you select the correct region.
Another account user shut down or terminated the instance
Check with other users of your account to see if they've terminated the instance.
The instance is running on another AWS account
Check other AWS accounts you might have to find the instance.
The instance was running, but a process or service shut down the instance
Some AWS services, such as AWS Auto Scaling, AWS Elastic Beanstalk, Elastic Load Balancing, or AWS Lambda, can be configured to start, stop, and terminate instances automatically. See the service documentation for more detailed information.
The instance was launched as a Spot Instance, and the current Spot price exceeds your bid