Power switch lights up

I have replaced the motherboard, battery and fan assembly (re-applying the thermal grease), however the computer does not boot up. When the power switch is pushed, the fan attempts to run (like a qick “pulse”, with slight movement of the fan blades), however it does not boot and there is no indication on the LCD screen. I have also disconnected the batter from the motherboard and attempted to re-boot (again the slight pulse of the fan)- no results; I have disconnected the CMOS battery and reconnected it (and made sure it was seated properly) Abain, “no boot”. I have double checked all the cables I had removed when replacing the motherboard. Is there a specific cable, if not properly reconnected, that could cause the problem I am experiencing?