Computer wont start

im trying to fix this computer it wont start. i tested the power supply with a tester and it works and i even put it in another computer just to be safe. i disconnected everything except the main power to the mother board. and the computer still wont start. there’s a green light on the motherboard and somebody told me that if the lights on its not the motherboard i dont know if its true but any suggestions would be helpful

thanks in advance
Nick C