Nginx is down

Nginx sits between HAProxy and gitlab-workhorse, which means a wide-spread of nginx instances being down could bring GitLab.com down completely or put a lot of stress on unaffected nodes.

Invalid configurations

Nginx can be down because of invalid configurations. Check /var/log/gitlab/nginx/current for lines similar to [emerg] invalid parameter "xyz" in /var/opt/gitlab/nginx/conf/some-file.conf:8. If found, then a recent update has been made to Nginx Chef attributes and they need to be reverted or rectified, followed by running chef-client on the affected hosts.