I could not access my lightsail instance and after a re-boot, I now get a 'welcome to nginx!' message

I think we can place your application content in the Nginx document root.
"welcome to nginx!" is the default Nginx page.
This appears when content has not yet been placed in the document root.

The Nginx document root is probably "/usr/share/nginx/html", so try placing your content in this directory.
Incidentally, if the directory is different, please check the Nginx configuration file named "nginx.conf" in the directory "/etc/nginx".