The DNS server is not responding to clients.
Cause: The DNS server is affected by a network failure.
Solution: Verify that the server computer has a valid functioning network connection. First, check that related client hardware (cables and network adapters) are working properly at the client using basic network and hardware troubleshooting steps.
If the server hardware appears to be prepared and functioning properly, check that it has network connectivity by pinging other computers or routers (such as its default gateway) that are used and available on the same network as the affected DNS servers.
Test a TCP/IP configuration by using the ping command.
For more information:https://technet.microsoft.com/en-us/library/cc787724%28v=ws.10%29.aspx