After an upgrade or update of Ubuntu 14.04 or higher, it's possible the console will remain black, even after pressing the 'ALT' key for disabling the screen protection.

In that case the 'channel' your console is displaying (the TTY) is not the correct one (usually TTY7 instead of TTY1). Because of this the screen will remain dark. This is because Ubuntu expects the GUI to be available (which is on TTY7 by default), but the server based version will not have a GUI unless manually installed. To solve this issue you can use the keyboard combination 'CTRL + ALT + F1' to switch to the correct TTY. F1 corresponds with the number of the TTY, so using F2 will switch it to TTY2, etc.

It is possible to use the GRUB boot settings to configure it so that Ubuntu will use TTY1 by default after a reboot. For more information you can consult the page on AskUbuntu regarding this.