HP PC will not post, no beep codes, no nothing.

[PwnFx]

I was given a "non-working" hp computer that I wanted to figure out the problem and give to my step son. It will not post. I get no display on the monitor, no beep codes.
I've tried using a different PSU, I've tried using a graphics card, I've booted with one stick of ram, with no sticks of ram, I've reseated everything. I've reseated the CPU, reapplied thermal paste,
I've reset CMOS, both using pin jumper and pulling the battery.

I've done everything I know of to troubleshoot.
The monitor isn't bad, it's the monitor I'm using right now.

What could be the problem?
What further steps can I take to troubleshoot?

Thanks in advance.

e/ Is it safe to assume it's a dead mobo?

 

[Chicano]

If the second PSU is known to be working normally, the motherboard is proably dead. If in any doubt about the PSU, try a third one.

Further troubleshooting steps would be:
Checking that fans run normally, specially the CPU cooler fan, and that all LEDs turn on... that would be: motherboard leds if any, and the front panel leds, particularly the HDD led which if flickering would indicate HDD, motherboard & CPU activity.