If you’re running Server 2012 R2 in a vSphere 5.5 environment with version 10 hardware then you may have been greeted by this sight after a reboot (typically after installing Windows Updates):
Server 2012 R2 Boot Screen

Well you’re not alone, it’s a known issue with v10 hardware and multi-vCPU Windows 8/8.1 and Server 2012/2012 R2 VMs as documented here: http://kb.vmware.com/kb/2092807 – in essence if your VMs have been more than 2 or 3 months without a power cycle (either full Power Off or hard Reset) then there’s a very good chance it’ll hang on start-up after a soft reset.

The fix is fairly straightforward but you have to apply it to every affected machine as there doesn’t seem to be a way to set it globally – hopefully this will be fixed in vSphere 6 when it arrives.