> Check the video cable and video source.
The HP monitor is somewhat like a tuner on a TV -- you have to select an "active" TV channel, to get a picture. 
Similarly, on the HP monitor, use the OSD (On Screen Display) buttons on the edge of the monitor to select from the various "sources" on the monitor, such as VGA, DVI, DisplayPort, HDMI, S-Video. One of those inputs is connected to the output from your computer.