Instance needed rebooting

When the system is completely offline, no metrics are sent to CloudWatch metrics, so I believe it is possible to check from there.
I believe there will be a log of other shutdowns in the system log.

Have you looked at the instance metrics tab? https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-viewing-instance-health-metrics CPU metrics, Network traffic, or status checks might give you that information.

If your instance exceeds its burst capacity, you might see temporary unavailability. If this is a persistent issue, you can create a larger instance from a snapshot to check if this resolves the issue. https://lightsail.aws.amazon.com/ls/docs/en_us/articles/lightsail-how-to-create-instance-from-snapshot