What PSU and how old is it? What CPU cooler?
I would pull the CPU and check the socket very closely for bent or broken pins.
Have you tried posting with only one stick of memory in each individual slot?
You could have a short, posting on a cardboard box (outside of the case) is another troubleshooting measure.
Have you tried clearing the cmos or removing the board battery for awhile?
You'll want a cpu installed otherwise the motherboard led will give code 00.
When you try to start it, are you getting code 00 with no cpu installed? If you're not getting a code at all this could point to a faulty psu or motherboard.
Double check the 24-pin main power cable is secured.
You could try flashing the bios with bios flashback.
No code from the motherboard is not a good sign, would try another psu or motherboard if you have access to either.
Could try bios flashback as a last ditch effort.