First, I thoroughly recommend reading Ben Armstrong's article, "Understanding Networking in Hyper-V," for an explanation of how networking changes on a Hyper-V enabled host. He explains that the real NIC on the host is converted to a virtual network switch and a new virtual NIC is created on the host to access the network using the virtual switch.
Sometimes networking doesn't get setup properly on the host when the Hyper-V role is installed. Here's how to fix it:
1. Open network properties of the NIC and uncheck all the connections on the adapter (Client for Microsoft Networks, IPV4, IPV6, ...everything) and click OK. Obviously, this will disrupt network communication to and from the server, so plan this work accordingly.
2. Click Virtual Network Manager in the Hyper-V console and create a new external network bound to the real NIC. This will convert the real NIC on the host to a Microsoft Virtual Switch and create a new virtual NIC for the host to use for its network adapter (see step 4).
4. Back on the host, you will now see two NICs in the Network Connections window. Local Area Connection (the original real NIC) is now a Microsoft Virtual Switch and is using only the Microsoft Virtual Network Switch Protocol. You will also see a new virtual NIC (usually named "Local Area Connection 3," in my testing). I've renamed it here to Virtual Local Area Connection.
6. Verify that the host has network connectivity, and then check your VM guests for connectivity.
Let me know if this helps you.