I had a strange situation last week. I deployed two Windows Server 2012 R2 Datacenter VMs in Azure and tried out the new management experience through the preview portal. At the same time I had done some VM configurations in the current portal. Maybe that was the reason why one of the VMs became not activated. I don’t know.
But no worry Microsoft Azure is using Windows Server 2012 R2 Datacenter for its Hyper-V hosts and with Windows Server 2012 R2 Microsoft has introduced the Automated Virtual Machine Activation key. So it is very easy to reactivate the VM. Grab the Windows Server 2012 R2 Datacenter AVMA key from the TechNet site and enter the key into the change product key window.
Voilà the VM is activated again.