@Dubitout
Sorry, but with your PC not even starting, there is nothing we can do -- as we could only provide links to diagnostics, and if your PC will not start, it could not run those.
The green LED means there is power the motherboard -- but the lack of any fan noise and the lack of any other lights or screen display indicates serious possible hardware failures.
You would need to get it examined to determine why it will not run.
For repair estimates, you would need to contact an HP Repair or Service Center.
If you live in the U.S., here is a link to the HP Service Repair Centers:
https://www.service-center-locator.com/hp-hewlett-packard/hp-hewlett-packard-service-center.htm
If you live outside the U.S., here is a link to HP Service Centers, by Country/Region: https://support.hp.com/in-en/service-center
Good Luck
I am a volunteer and I do not work for, nor represent, HP