re-seat everything. re-seat the ram (cuz that's easy), re-seat the CPU if you have to. did you apply thermal paste? is the cooler installed correctly?
unplug all peripherals (except keyboard, mouse, and monitor).
unplug all storage, like SSDs and hard drives.
unplug the video card, and plug the monitor directly to the motherboard.
plug in only one ram module at a time, and, try different ram slots.
plug in only the most essential things: motherboard, CPU, RAM, all powered up from the Power Supply, and that's it.
also, unplug front panel connectors, and power on using your screwdriver to short the pins that say pwr btn +-
another idea, take out the small CMOS battery for ten seconds, unplug everything for ten seconds, power on the motherboard (with everything unplugged, it won't work, but will discharge any left over electricity still in its circuits, as well as reset the CMOS. Then, put the the battery back in, connect the mobo and power on.
then, troubleshoot, plug in one extra component at a time, to narrow down the problem.