Computer display wont turn on?

I disembled my whole computer and cleaned evey part with a cotton swab, then i re-embled it all back and now it turns on but no display.There's no beep code when i start it, the monitor shows its got signal.I have tryed 2 diffrent monitors,power supplys,graphics card and onbord graphics, also tryed pulling everything except the cpu and power out. Please help me.