If I understand you correctly, the psu, vid card, cpu, and memory can each be individually installed in another PC, and that PC works.
So, the problem is in your motherboard. Reset CMOS (pull plug on psu, remove battery, press the case power on several times, use the clear CMOS jumper on the mobo if you have one, wait ten minutes, put battery back in).
If that allows you to get into BIOS, restore defaults, etc.
if not, the mobo is gone.