Cannot connect to lightsail instance even though it is running

There are several potential reasons why you might be unable to connect to your Lightsail instance even though it's running. Let's explore some possible causes and solutions:

1. Instance startup: If you recently restarted the instance, it might need a few minutes to fully initialize. Wait for a short while and try connecting again.

2. SSH key issues: Ensure you're using the correct private key that corresponds to the public key on your Lightsail instance. If you're using the default key pair, you may need to download it again from the Lightsail console.

3. Key file permissions: If you're connecting through PowerShell, check that the permissions on your private key file are correct. The file should be readable only by you.

4. Firewall or security group settings: It's possible that the instance's firewall or security group settings have changed, blocking incoming SSH connections. Verify that port 22 (for SSH) is open in your instance's firewall settings.

5. Network connectivity: There might be network issues between your local machine and the Lightsail instance. Try pinging the instance's public IP address to check connectivity.

6. Instance state: Although you mentioned the instance is running, double-check its status in the Lightsail console to ensure it's fully operational.

7. Recent changes: If you made any recent changes to the instance configuration or networking settings, these could be affecting your ability to connect.

Regarding the IP address change when stopping and restarting the instance, this is normal behavior for Lightsail. When you stop an instance, it releases its public IPv4 address. To maintain the same IP address, you should use the "reboot" option instead of stopping and starting the instance.

If you continue to experience issues after checking these points, you may need to contact Amazon Lightsail support for further assistance. They can help investigate any potential account-related or infrastructure issues that might be causing the connection problem.

Remember, it's crucial to have a backup plan and perhaps consider using Lightsail's snapshot feature to create regular backups of your instance. This can help you quickly recover your website in case of persistent connectivity issues.

Sources
Instances - Amazon Lightsail
Resolve connection errors with Lightsail browser-based SSH and RDP clients - Amazon Lightsail
I can not connect my Lightsail instance by ssh from my terminal ? | AWS re:Post