You recently install a Windows Server 2008 R2 and patched SP1. The server (typically HP server) has multiple Intel CPUs, e.g. two Intel (R) Xeon(R) CPU 5160 3.00GHz.
You then enable the Hyper-V role, it required a reboot. But after reboot, the system didn't boot up, it just stayed in black screen.
Although the CPUs are identical, the revision or stepping are different. If one is revision 'CD' (stepping is 6) and the other is revision 'B9' (stepping is B), the system is not booting up with Hyper-V enabled.
Replace the CPU and make sure all CPU are identical with same revision and stepping.
To check CPU information, run WBEMTEST and enter the query "Select * from Win32_Processor" (without the quotes) and click on the Apply button.