Greetings,
Welcome to the forum.
I am not a HP employee.
What happened when you restarted the PC? Any video at system power on?
Or is the system dead?
If you get any video at system restart you can try running HP Diagnostics (Link). Scroll down this link to see HP UEFI Diagnostic details.
Have you tried a different monitor? I would do this to eliminate a monitor problem. I don't think the monitor is the fault but you never know.
It sounds like a PC hardware problem but could be anything.
Check all connections to the PC. Disconnect printers and any other unnecessary peripherals.
Regards