Won't power on after replacing HDD, no lights, no screen, no fan sound

My hard drive in an old HP 15 r218na died recently, so I went to change it, this involved a more thorough strip-down of the laptop than I remembered from previous cleaning of the hardware so as to reach the drive.

I have re-seated all the ribbon cables, I have put new thermal paste on the CPU, I have removed and replaced the BIOS backup battery, but I can't get the machine to power on. I was wearing an anti-static wrist strap the whole time, was not wearing anything woolen, and the UK isn't a particularly static prone place at this time of year.

It is definitely getting power, a little light on the AC adapter's input port comes on when I plug the adapter in powered on.

But when I try to power the laptop on with the power button all I get is a blink of another power light on the machine, for about 6 seconds, then nothing. Total silence, no sound of a fan starting. No pre-boot screen appearing.

I checked with a mutlimeter and when the ac adapter is in there is 3.3Volts on some of the ribbon cables to the power button circuit board, so that is getting power. It clearly senses the pressing because when the button is pressed there is a 6 second burst of light on the bottom left corner (near the headphone jack) white indictor LED.

Everything I tried with the Ac adapter in I also tried with the battery in, no success.

I have stripped it down and re-seated the cables several times now.

Please help.

Can anyone find full proper schematics of the laptop, HP's webpages referencing this laptop model are useless, dedicated solely to fixing things which don't matter until after an OS boots anyway, so I can't find anything worthwhile there. Or any detailed info about boot sequences and light flashing times?

I get about 6 seconds of light on the LED starting from the moment I RELEASE the power button, no beeps, no screen backlight, nothing else, not even the fan starting or the hard-drive's (new one of exactly the same make and model as original fitted) whir.

Thank you

P.S. I have all the kit for fiddly soldering and am good at it, but that's no use unless I know that a ) there is a damaged electrical part, and b ) which one and c ) have an actual schematic and explanation of what happens during pre-boot powerup

P.P.S. The orange light on the keyboard under the wi-fi enable key (F12) also flashed for the 6 seconds that the power LED flashes. But no backlight appears in the screen, and the light under the power button itself never lights up.

Can't understand it but, the laptop itself magically started working again after a few more cycles of disassembly and re-assembly. No clue how, but it runs now and seems to be back in good health. Didn't do anything differently between the times it failed to power on when reassembled and the time it just started working again, but it works now.