this may sound patronising, but im a first liner, so:
Is your power supply properly plugged in and switched on?
Have you made sure that your 24 pin and 8 pin sockets have been connected properly?
Have you made sure that any optional power is supplied?
Have you tried only running the system with 1 DIMM in the a1 and b1 sockets?
Have you tried removing the GPU and seeing if it could be a potential power issue?
Have you praised the sun?
And in all honesty, have you tried dismantling the machine and testing the mobo/cpu with cooler and one DIMM outside of the case?
Taking everything out, get a cool drink or hot - whichever you prefer, take a quick break, watch something on a laptop and then come back to it.
Edit:
If it doesnt work - panic, run, cry, nap and make someone else do it.