This problem occurs frequently, my host Host version is Windows Server 2012 R2, the older version, on the new Windows Server 2019, we found no problem.
Symptom is Hyper V virtual host suddenly can not access, you can also open the virtual machine host through Host host, IP address, and then configure the network card is normal, ping is illogical.
General IT HOST will simply restart the server, the problem can be resolved.
As two like IT, I do not do it, put the station in question a virtual host for a virtual switch on immediately better. Do not tell me your server two network cards, usually when I purchase the server requires at least four network cards, virtual switches do, I'll do at least three, to allocate the use of virtual card based on the number of virtual machines.
You want to show off about technology, do not use GUI way to operate, Administrator, open Powershell.
GET-VMNetworkAdapter, and then Set-VMNetworkAdapter
Command does not know how to use it, Oh, look at the document. English can not see it, give you a dictionary, and then give you a how the dictionary.
https://docs.microsoft.com/en-us/powershell/module/hyper-v/set-vmnetworkadapter?view=win10-ps
To learn solution to the problem, the problem will be easily solved. Of course, the command line can be solved, the less points the mouse, or peers will look down on you, steal your division, you learn the arts, but also ruin behind you.