this happened to me recently with an 8RGA+. I think this is a grounding problem.Take everything out of the case. Then place the motherboard on something wood or plastic. Only hook up the power supply and one stick of ram, Video card and of course leave the cpu and heatsink installed. Then turn the system on it should boot , if not it is a bad board
You can also try resetting the cmos by powering the system down, turn off switch on PSU, then remove the CMOS jumper and move it to pins 2-3, wait about 20 secs then put it back in pins 1-2, that might work as well.
You can also take the cmos battery out and leave it sit out for a while to make sure the cmos is clear then reinstall it and that might get you to post
If it is a grounding problem you need some plastic or cardboard washers between the board and the screw heads, I used some Electrician's tape, on the particular build I was having problems with closed box no window. On my windowed builds I use aluminum thumb screws and brass stand offs, too alevaite the grounding problems
Hope this helps