Greetings,
Welcome to the forum.
I am not a HP employee.
Your PC has a Intel 8700K processor. This processor has Intel integrated graphics.
If your PC has an Nvidia 1080 then you have two different places to connect a HDMI cable to the PC.
You may be connecting the HDMI cable to the wrong HDMI port at the rear of the chassis.
The Intel HDMI port is located on the rectangular I/O shield where other connections such as audio, ethernet, and USB are located. The 1080 has separate HDMI or Display Port connectors that may be located above or below the I/O shield. The location depends on how the MB is mounted in the chassis.
Look, the graphics card could be bad if you are connecting correctly.
Try the Intel HDMI port to see if you get video. But you will have to remove the 1080 from the system to see if Intel graphics works.
Regards