I have an MSI K9N6SGM-V motherboard, and recently when I power on my system the fans and the lights on the case come on, but I don't get a visual, nor does the number lock on the keyboard light up. At first I thought that the on board GPU had died, but I can't even ping the system.
Yet if I press a key on the keyboard, then the system will power on and I get a visual. I've tried setting the optimized defaults in the BIOS but that doesn't change anything.
Powering on by itself is also normally caused by a setting in the BIOS in the power management section as is wake up on keyboard. The two could be related, which is why resetting the CMOS memory might clear the problem.