Seems like the 'obsolesence time bomb chip' has kicked in.
Give up now and save the frustration.
I had similiar, replaced every component twice over from other computers i knew had worked well in past.
No combination of power supply,ram,hard drive,motherboard or cpu would work.
Ended throwing the whole lot away and started a new.
Might be able to recover data from the hard drive.
Could try bridging the two pins the start button is connected to.
That would illiminate the button.