Gateway NV55S will not boot up. Does anyone have an answer

I was given an NV55S Gateway and I replaced the MB and CPU. I still cannot boot this laptop and have the start button and whatever else to troubleshoot... I have a power light when plugged in but nothing else. Can anyone Help me with this? I've already spent more than I should with MB and a new A8-3510MX CPU, AMD's FS-1 socket set second from the tops in performance available, and have tried putting the old A6-3400 series CPU back in it with no luck. I'd like to win one for a change..Help!