From Microsoft: Service Restored
Preliminary root cause: A configuration change was deployed, which inadvertently caused the toolbar UI from not loading as expected.
We're performing a post incident investigation to fully understand the underlying cause of the issue.
We received positive feedback that users had been able to use alternate methods (Teams web and mobile versions) to workaround the issue.
While we work to mitigate the problem, users could have used keyboard shortcuts as a short-term mitigation. Additional information is located here: https://support.microsoft.com/en-us/office/keyboard-shortcuts-for-microsoft-teams-2e8e2a70-e8d8-4a19-949b-4c36dd5292d2