Always Free OCI instances stopped and cannot start

Summary:

I have an Oracle Cloud Always Free tenancy with four OCI Compute instances in ap-sydney-1.

All four instances were actively being used yesterday and suddenly changed to STOPPED. They are now shown as STOPPED and cannot be started.

When attempting to start them, OCI returns:

"Instance [OCID] is disabled and will not accept any action requests. Please contact customer support to reenable."

The OCI Console also displays sometimes:

"To use this service or resource, you must upgrade to a paid account."

My tenancy has NEVER been upgraded to paid.

I have received no security/abuse notification, no maintenance notification, and no email explaining the disablement.