Black screen, No POST, green led, Fan load - HP Compaq 8200

[slash17]

Hi all
Just few days ago I brought an HP Compaq 8200 (this one) from my company at home, to recover some files on it (it has been off for the last 6 months).
It started up without any issue and worked just fine for a couple of days, with no restart, until I decided to shut it down on friday evening.
From then on, I was not able to start it again!

The behavior is the following:
- all cables connected, aux led is green, no blinking
- when i press the power button the power led turns green (no blinking) and the fans start at normal speed
- no bip, no red led, no blinking, but nothing happens on the display (connected via VGA)
- after about 50-60 seconds, fans start spinning more and more fast, but again, nothing happens on display and leds
- then I shut it down by holding the power button for some seconds

As far as I know, nothing happened from when I shut it down (friday evening) and when I tried to start it again (saturday morning).
It was connected to an APC Back-UPS.
It was also connected to the modem via ethernet cable, but the modem is working well so I guess no power issue happened there.

I tried the following:
- remove dust with a vacuum cleaner
- reseat ram
- leave just one memory stick
- reseat cpu
- reset CMOS, both with the dedicated btn and by removing the battery
- replace the CMOS battery (even if it was still 3.03V)
- disconnect every non-necessary stuff, leaving only mb, cpu, 1 ram stick, front panel leds and button
- remove all ram sticks -> nothing changed! I was expecting some bip and red blinking leds but nothing, just exactly the same behavior
- remove the cpu power connector (on the top left of the CPU itself) -> red blinking pwr led and 4 bips
- measure the 12vsb cable: ok (11.6-12.1V)
- remove all the power cables, manually start the PSU by short-circuit pin 3 and 4 of the power-on connector, measure the 12v cables: both 12v CPU ok, also 12v main ok

Everything there tell me that it's an mb issue...but I really cannot accept to be so unlucky that it happened right when I brought it at home!

Any suggestion here?
I've read about a "crisis bios recovery" feature on some HP mb, but I can't find anything on the 8200's manual

[itsmyname]

Can you temporarily connect a different power-supply? 

A power-supply must output a few different voltages.

Maybe, it still is sending the correct voltage for the LEDs, but not the correct voltages for the motherboard or the auxiliary power to the processor?

More difficult: swap-in a different processor, to see if that makes a difference.

Does your company have an identical "spare" computer in their I.T. Department?

If so, bring it home, and mount your disk-drive into that "spare", and boot from it.