Keywords: LAMP/MAMP/WAMP - AWS - Technical issue - Connectivity (SSH/FTP)
The issue is exactly (almost) like the one mentioned here but the solution doesn’t work anymore.
I have waited for more than an hour after running custom script with “create new instance” from broken ssh snapshot. Still the server replies with Connection Refused.
Also tried from lightsail console, it says UPSTREAM_NOT_FOUND 
The only difference in my case is that the website has stopped working altogether.
For your information, I followed following tutorial to update 16.04 to 18.04