Earlier today, Microsoft Teams wad down for a couple of hours and while it caused some disruption for users, the actual reason behind the issue was more interesting – especially as it is coming from such a large company.
According to the 365 Status Twitter account, the SSL certificate had expired for allowing the Teams app to talk to Microsoft servers.
We've determined that an authentication certificate has expired causing, users to have issues using the service. We're developing a fix to apply a new certificate to the service which will remediate impact. Further updates can be found under TM202916 in the admin center.
— Microsoft 365 Status (@MSFT365Status) February 3, 2020
Teams went down at 9am Eastern Time, the fix was announced at 11:20am Eastern Time and then by 12pm Eastern Time Teams was back up for users.