Replaced mobo on Dell XPS m1530 laptop and now it won’t start

I replaced the mobo with another provided by Dell and now the laptop won’t power on. The fans don’t run and the power button lights up for a half a second. Dell Tech Support is sending out another mobo for me to try for the customer but I need to know what’s going on. The previous mobo (the one I replaced) worked so I replaced it back into the system now it won’t start. I don’t know if I shorted something out or if the case is shortening out the mobo. I just need a little help because I have to go back Monday and try to repair the system again.