I always try the simple stuff first
. I have 3 working computers so it's easy for me to swap out parts for testing.
No POST, no BIOS beeps?
I would have first checked out the PS (by simply temporarily replacing it with a known good...and by putting the one from the system with problems in a known good system).
Satisfied that the PS either works or doesn't work...I would have moved on to the hard drive. Check connections, connect to known good system as a secondary drive, see if I can see files on the HD. If drive is good, reconnect.
Then, I would have removed the memory and reseated same, a stick at a time individually (later both). Try to boot. The chances of both sticks being bad at the same time, well I haven't experienced such yet.
If all of these yielded nothing, then I would (by default) conclude that the motherboard was shot. Only then would I think of replacing it.
I guess I have to ask...if a repair shop determined that the MB was bad, did they install the replacement? If so, is it not covered by some type of warranty?
Did the system perform properly after taking it to a shop? Ever?
Was there evidence of power to any of the fans? To me, it's inconsequential if I see lights, those usually don't draw much compared to the rest of the system. The only thing a flashing light would indicate to me is that...some part of the motherboard is functioning or there would not be enough power to create the flashing light.
This is out of my league...better wait for the persons who deal with this sort of thing every day and who donate time to this forum
. Have a little patience.