We had the same issue on Friday. Our monitoring system that uses the API showed all client devices as unavailable with a message "No Data returned" . Had to put the devices in maintenance as it was generating SNOW Incidents.
We changed the API key on one credential and it seemed to work. But not sure why it stopped in the first place. API keys were generated in January 2023.
The second organization started working mysteriously this morning at 9.33 a.m.
Spent all day today getting the devices out of maintenance and running the API discovery tool.
Case was logged with Meraki and they said to call for Live Troubleshooting but since it started working did not bother.