|It certainly can be the PSU, but not necessarily. You can try reseting BIOS to default settings to see if something changes.|
I recently had a system with the same issue as yours, and the problem turned out to be leaked motherboard capacitors. So you should open the side of your case and search for such caps.
Some pics of what you should look for...
Hope this helps.
btw, the MB diode is always on, as long as the PC power cord is plugged in.
Edit: I am assuming that with 'nothing happens' you mean that the fans are spinning but nothing comes up on the screen.
Google is your friend
Have a GREAT day!