Dell inspiron 5150 turns on but wont boot

Hello,
Can someone please help I recently purchased a bunch of dell inspiron 5150 Well three I purchased a new mother board for one and that one works perfectly now. But the other to for what ever reason dont work I've changed parts on them and now they both turn on the fans turn on and they stay on but nothing comes up on the screen I tried hooking them up to an external monitor and pressing FN, F8 but they wont even turn on a external monitor. I'm at the end of my rope and really dont know what to do If anyone might know your wisdom is really appreciated. Thanks for your time. Frank