Screen, mouse and keyboard dont power

Hello,

So Im having a hard time figuring out what to do. I have a cpu , ram, ps, hard drive and motherboard that work (they were all checked on other computers and they worked) but when I put them all together they seem to not like each other. Everything turns on, nothings beeps but my screen stays black and the keyboard and mouse dont seem to even get power to them. Any ideas whats wrong? They are all parts pulled from other computers but compatible. Do I need to install drives on the motherboard or something? Im at a loss! Also my mouse, keyboard and monitor do work.