What happens to my existing provisioned instance when AWS runs out of instance capacity?

I am using the g4dn.xlarge instance types (on-demand), distributed across the AZs: us-east-1a,us-east-1b,us-east-1c,us-east-1d,us-east-1f. They work fine until one fine evening I cannot power on any of these instance types and get a message via the API: "Temporary insufficient AWS capacity for instance type g4dn.xlarge in the following zones: us-east-1a,us-east-1b,us-east-1c,us-east-1d,us-east-1f"

I am not using any Capacity reservations here. I would like to know if these instance types were given to someone else when I tried to power them on? How does this works as I see this issue recovers on its own?