Running a large query under PhpMyAdmin was generating a 400 Bad Request error under Nginx.
To restore the front end required the cookie for the PhpMyAdmin domain to get cleared from my browser cookie cache.
It seems that the Nginx crash was corrupting my cookie cache and crashing nginx.
Adjusting the domain vhost file with the following got things working.
server {
# ...
large_client_header_buffers 4 32k;
# ...
}
Since in my case the Vhost is generated through ISPConfig panel I added the above line to the VHost setup for the server config.