Website not loading

Based on your description, you're experiencing multiple issues with your Lightsail WordPress instance. Since you can't access the site via domain name or IP address, and you're also unable to connect via SSH with an "upstream error 515," this suggests a more serious problem with the instance itself rather than just a DNS issue.

Here are some troubleshooting steps to consider:

1. Check the instance health metrics in the Lightsail console. Look at CPU utilization, memory usage, and disk space. Your instance might be running out of resources, which could explain why it's not responding.

2. Try a snapshot restore if you have a recent backup. Since you can't access the instance via SSH, restoring from a snapshot might be your best option to recover the site. This has worked for other users with similar issues.

3. Consider upgrading your instance size. If your WordPress site has grown or is receiving more traffic, your current instance specifications might be insufficient, leading to performance issues or crashes.

4. Check if there's a pattern to the downtime. You mentioned the site previously required nightly reboots to keep running. This suggests an underlying issue with your WordPress installation or server configuration that should be addressed for long-term stability.

5. If the snapshot restore doesn't work, you might need to create a new instance and migrate your content. You can create a new instance, install WordPress, and then import your content from a backup.

The "upstream error 515" when trying to SSH suggests that the instance might be in a failed state or experiencing severe resource constraints, which is why basic access methods are failing.
Sources
Lightsail Wordpress site not accessible | AWS re:Post
Webpage is loading slow | AWS re:Post

Hello.

Which plan are you using for your Lightsail instance?
If you are using the Lightsail plan with the smallest specifications, even a small amount of CPU load will use up your burst credits and your instance will operate at baseline throughput.
This may occasionally cause the website to become unstable.
If you find yourself experiencing high CPU load and consuming burst credits, I recommend you change your Lightsail plan or migrate to EC2 and host your website on an appropriately sized instance type.
https://docs.aws.amazon.com/lightsail/latest/userguide/baseline-cpu-performance.html

Have you checked the instance health metrics?