I am guessing, that you have your board laying flat, and the computer is unhooked from all the cables.
first thing is to make sure that all your sockets are clear of dust by blowing high pressure air to the board (dust is a killer of boards).
ok onto the next step turn the computer over to check for lose screws that might have dropped onto the motherboard
ok next is that all of your cards are clean dust free you can take a static free cloth or high pressure air to clean/clear your cards then replace all the cards and memory sticks
if you have a extra power supply that you know is good or a friend has one thats good try hooking that up first.
If your computer fire's up now then you can try your power supply if dosen't work then you have boiled your problem down to your power supply and that it should be replaced