UPSTREAM_NOT_FOUND [519]

Hello.

Create a snapshot from your Lightsail root volume and create a volume from that snapshot.
I think you need to mount that volume on a healthy Lightsail and check the system logs etc. stored on the volume to troubleshoot.
If you determine that it cannot be restored after troubleshooting, you will need to take measures such as extracting the necessary data from that volume and recreating it using another Lightsail.
https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-create-an-instance-root-volume-snapshot

Thank you for your response. I should have mentioned that my instance is a Wordpress instance, so I am not sure that your instructions apply to me or not? Is this different than simply clicking the "Create snapshot" button? Thank you very much.