To upgrade be sure to follow the steps prescribed by Gitlab.
If you cannot connect after upgrading try the tips located at this page.
https://github.com/gitlabhq/gitlab-public-wiki/wiki/Trouble-Shooting-Guide
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.
To fix
rm /var/opt/gitlab/gitlab-rails/sockets/gitlab.socket
gitlab-ctl restart
Try your site again, https://gitlab.example.com