To upgrade be sure to follow the steps prescribed by Gitlab.
If you cannot connect after upgrading try the tips located at this page.
Especially this issue might be affecting your system.
Nginx can’t connect to Unicorn Socket
Problem: Everything seems to be running correctly but you still get a 502 error from Ngnix and when you check the logs, you see a
Connection refused error.
Solution: Delete the
gitlab.socket file, located at
/var/opt/gitlab/gitlab-rails/sockets/gitlab.socket and restart GitLab with
gitlab-ctl restart. The file should be recreated after a few minutes and you should be able to access your GitLab instance again.
Try your site again, https://gitlab.example.com