I encountered the same problem after upgrading my droplet to Ubuntu 16. I solved it by adding a systemd service configuration for ghost.

I created the following file: /lib/systemd/system/ghost.service

With this content

[Unit]
Description=Ghost: Just a blogging platform
After=network.target

[Service]
Type=simple
WorkingDirectory=/var/www/ghost
User=ghost
Group=ghost
ExecStart=/usr/bin/npm start --production
ExecStop=/usr/bin/npm stop --production
Restart=always
SyslogIdentifier=Ghost

[Install]
WantedBy=multi-user.target

This is based on the following systemd script: https://github.com/TryGhost/Ghost-Config/blob/master/systemd/ghost.service & https://www.digitalocean.com/community/questions/failed-to-start-ghost-service-unit-ghost-service-not-found