If your install of Windows 10 is suddenly no longer activated, Microsoft is aware of the issue, but we don't know when this problem will be resolved.
Microsoft has introduced a new way to activate Windows with Windows 10 called digital entitlement and now the company has posted new activation guidance.
Check out this post and learn to use Automatic Virtual Machine Activation (AVMA) to auto-activate the guest OS installations of Hyper-V VMs!