Computer start but can't boot

Hi guys, I have a serious issue with my computer.

It's been until.. two, three months that sometime, my computer turn on, but nothing boot.
My monitors says, "No Signals", and nothing else, everything is working, USB (when I click on my mouse, it light up, but when I release it, it switch off), I've tried to change the CMOS Battery,nothing still happen.

I've tried without graphuc card, all the connectics (VGA, HDMI & DVI), other cables, other monitors, still the same. I don't know what to do now.. I'mm pretty lost.

This is why i'm here, so thank you for your answers in advance.