Can't connect to Lightsail instance(Upstream_not_found [519] error)

Hi, I have been running a website for last three years on Lightsail. The instance was configured with Ubuntu(possibly 20.04 LTS, not sure), and I added nginx and django on it by myself.

I have found myself able to connect to the website(via HTTPS) a month ago. Haven't checked for SSH connection at that time though since I didn't need to.

Today I found that I can't open the website and even can't connect via SSH. Terminal only displayed connection timed out error. Connect via browser on Lightsail console shows upstream_not_found 519 error.

Things I have done are these;

First, I have tried stopping the instance and restarting it.

Then I created a new snapshot of the instance and used that to make another Lightsail instance, with and without starting script resetting sshd. I also started new Lightsail instance with higher grade plan in case of storage/memory shortage.

I have tried to restore through snapshot created a week ago(the oldest I could get) to start new instance, with all possible options mentioned above.

I have exported the snapshot to EC2, but still not able to ssh into it.

Metrics graph shows that instance status check have failed since 26th September, around two weeks ago. System status check had no failure at all. I haven't touched anything for last several months on ssh console.

running aws lightsail get-instances returns empty list.

{
    "instances": []
}

I can't lose data in this instance. What should I do?

Thank you for your help.