Greetings,
Welcome to the forum.
I am not a HP employee.
Nice troubleshooting!
I think you have a hardware problem; black screen with no mouse pointer. Do you have any PC fan or LED activity?
Possible causes are the power supply, a component connected to the motherboard, or a peripheral device connected to external I/O, or the motherboard/CPU. CPU failure is very rare.
Reduce the PC to only a monitor. Try to POST.
No POST means you will have to test each component.
This can get expensive as you may need known good replacement parts.
It is sometimes best to have HP Service or a local PC tech find the bad part.
Regards