I know I am not telling you anything that you do not know, but check and recheck all connections and mountings.
Check the setting of the clear CMOS jumper. Many systems will not POST when the jumper is set to clear.
Make sure that there is not a support post that is touching the motherboard.
I did not check the specs on the motherboard. Does it have on-board graphics?
If so, remove the video card and see if the system POSTS with just the on board graphics.
If the board has PCIe slots, it may not recognize a PCI video card.
I would also double check the voltage switch on the power supply. And, make sure you have all the correct axillary power connections made.
After checking all the obvious stuff, I start with the power supply. Either swap in a different supply, or plug a test plug onto this supply.
Then come choices on swapping out other components. Usually based on what I have on hand that is compatible.
Chas