If you have a power supply tester, you should probably use it (and you might want to borrow or get one if nothing else works). If the fans are spinning, a good bet is that some power is reaching the motherboard, so check that everything is seated correctly (I find it best to remove and reseat the components so you know for sure) and that everything is plugged in tight. If the motherboard has onboard graphics, try connecting your monitor to that as the computer may be set to boot as that being the primary graphics, in which case the computer may not use a add-in video card. If none of that works, check that the processor is seated correctly and the memory is installed by the motherboard manual's guidelines. Some motherboards are picky and won't boot if everything isn't in the right place, even if it is present. Also make sure that the basic stuff like the monitor is set to the correct input and the video cable is plugged in tightly (I know it is basic, but sometimes that is the stuff that gets people).