When you are using Xen Server to arrange virtualization, and you have Debian or Ubuntu guests, plus you are using a crypto disk inside these guests, your screen will turn black as soon as the guest system boots. This all happens in the console tab of XenCenter.
In specific it will go black without showing you the GRUB menu. Although, booting continues and you are asked for your crypto passphrase — but you will not see this, as your screen is still black.
Nevertheless, just fill in your passphrase when you have waited around 15 seconds after the screen went black. After hitting enter, you will finally see your session come to life. Now login and quickly change this behavior.
/etc/default/grub with your editor and uncomment the following two lines:
Save the file, and now run
root). Reboot, and you will finally be able to see that you have to put in your crypto disk passphrase.
(Came across this when working at Forwart.)