This could be several things, but the most common are RAM, MOBO, and PSU.
I'd try the following:
1 Stick in the DIMM slot closet to CPU
Clear CMOS via jumpers ; see page 1-16 of the manual.
Unscrew the MOBO and PCIe screws, dangle the MOBO away from anything conductive and supported by a towel. Try to start.
Pull all USB and Front Panel connections, and Pull any thing connected via USB -- yes the keyboard & mouse. With a screw driver short the PWR + Ground; see page 1-27 of the manual.
Update post from there.