Maybe your firewall rules changed? Do you have Omnibus with GitLab and Mattermost on the same server, or did you install them on separate servers?
Log into the machine that hosts Mattermost, and execute this command:
curl localhost:8065, which should result in some raw HTML displayed in the terminal. If you don't get a response then Mattermost is probably not running. You can check the Mattermost log file at
/var/log/gitlab/mattermost/current to see why it did not start.
If you see the HTML, then Mattermost is running. In that case, check the NGINX config to make sure that it didn't somehow get changed during the upgrade. And check your firewall rules to make sure that port 80 is open.
Did you do anything other than
apt-get upgrade? Was Mattermost running, then you did the upgrade, then Mattermost was not running? Or did you do other changes too?