Dell optiplex 745 tower won't boot

Hello experts,

I am really frustrated with my dell optiplex 745. I have been endeavoring to get on when I replaced the power supply but with no success. I have also changed the motherboard and again nothing to be happening. The dell powers up just fine and then the cooling fan goes noisy. There is a green light in the front panel show showing the numbers; 1, 3,  and 4.  I would be grateful if you can help me with this issue.