I get nothing on the screen at all. Not even a flicker.
IF you are not seeing the HP splash screen nor any Power On Self Test information or anything providing info regarding which keys to strike for BIOS (System Setup) or Boot Menu options, then......
You'll need to start troubleshooting, probably starting with the monitor itself.....
With or without the computer on, I would expect to see something on the monitor in the way of an OSD (On Screen Display) if nothing else, informing you of "no signal" or something similar along with some kind of indicator light, usually green or blue and then turning to amber if it is not receiving a signal, so.....
Turn the monitor off and then on and see what you see.
Test the monitor on another computer (if that is an option).
AS I mentioned above examine and confirm that both ends of the interface cable are firmly seated as well as the actual AC power cable which may or may not be built in or a typical quick connect on the monitor end.