System won't boot but cpu fan spins and lights come on

Help!
When I press the on button the cpu fan turns on just fine and same with the gpu, but nothing is displayed on the monitor. I've tried multiple monitors (which I know to be working) and serveral cables but still nothing. I connected my old mb with a different cpu and the same thing happens.
P.s. I've gone through most of the troubleshooting I can find online ie Clearing CMOS and swapping out ram and still the same thing happens. Please tell me this is simply something I'm doing wrong