|First tell us why you replaced the power supply.|
Then tell us what make and model power supply. Since these may be of importance.
Have you gone into your BIOS and set your graphics to PCIe or 'disable onboard graphics if PCIe graphics card is present'?
Did you make sure that you did not disturb the graphics card in the slot when you changed out the power supply?
Did you make sure you connected the 4 Pin CPU connector as well as the 20 or 24Pin motherboard connector?
The more details you can give, the better. Post back resuts.
You have to be a little bit crazy to keep you from going insane.