Microsoft Teams has been down since about 8:30 am ET.

[DelJo63]

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

About an hour ago, the company updated the status again, indicating it had begun deploying the updated certificate.

[edit] It *MAY* take 24 hrs for the cert to be fully distributed [/edit]