Hyper-V guest hung before booting

Following a server reboot we had one of the Hyper-V guests (Server 2012R2 in this instance) hang on the initial Hyper-V splash screen (black background, white text) with no sign of progress, i.e. no Windows boot or spinning dots etc. Hyper-V manager showed the guest as ‘Running’ but attempting to shut down or turn off the VM just produced an error or pushed into into a hung state of ‘Turning off’.

Investigation suggested the issue was with the initial Hyper-V configuration as removing the virtual NIC from the guest allowed it to boot without issue. Re-adding the same virtual NIC reproduced the problem, but adding a new virtual NIC (on a different physical socket in the host) brought back full functionality. Note that as far as the OS is concerned this is a whole new NIC, so you’ll need to manually reconfigure TCP/IP addresses if it was on static entered address before.