|What have you changed or added? Is it just the Power Supply? Have you connected up all the PSU cables correctly?|
First try and ensure that all your cables are pushed in fully, any memory cards are pushed fully home into their slots and all cards are fully slotted into their respective slots. Then try rebooting.
If that does not work then try the minimum hardware approach.
That is connect just one memory card, the graphics card and see if your system will boot up.
If it does start adding other cards one at a time. If it is a hardware problem the offending piece will be identified.
☺ When everything else fails, read the instructions.