This issue has been resolved by Microsoft.
Microsoft has determined that an authentication certificate has expired causing users who have logged out and those that are still logged in to have an issue using the service. Microsoft is developing a fix to apply a new authentication certificate to the service which will remediate impact. Microsoft has confirmed that reports of impact to other Teams features such as chat history visibility and Auto Attendant functionality are related to this authorization issue.
Next Microsoft update by Monday, February 3, 2020, at 4:00 PM CST
Users may be unable to access Microsoft Teams.
Microsoft is investigating an issue where users may be unable to access Microsoft Teams. Microsoft is reviewing systems data to determine the cause of the issue. This issue may potentially affect anyone attempting to access Microsoft Teams.
Next Microsoft update by Monday, February 3, 2020, at 9:30 PM CST