Greetings,
Welcome to the forum.
I am not a HP employee.
Okay, the power supply has a solid green light. You hear startup sounds like fans, the HDD spinning up, and the optical drive initiating.
Check/reseat your video cable connections to the monitor and the back of the PC. Are you plugging the video cable into the correct video out port at the back of the PC? Some PCs have discrete graphics cards installed. On board graphics may not work when you have a discrete graphics card installed.
Regards