A. Microsoft is aware of a problem with Hyper-V VMs and their network connections. If a Hyper-V VM is establishing many concurrent network connections, or there's heavy outgoing network traffic, the network connection on the VM could be lost. Also, the network adapter could become disabled. The only resolution is to restart the VM.
If you're experiencing this situation with your Hyper-V VMs, take a look at the Microsoft article "The Network Connection of a Running Hyper-V Virtual Machine is Lost Under Heavy Outgoing Network Traffic on a Windows Server 2008 R2-based Computer". Attached to this article is a hotfix that should be applied to the Hyper-V host.
This hotfix may resolve your missing network adapter problems.