That could be caused by a myriad of reasons.
To narrow it down you'll need to give us more detailed information.
System specs
Did you change any hardware/software before it startd happening
Have you tried booting up with just the bare minimum,HD,floppy RAM

If absolutely nothing happens when you press the power button it would seem like the board isn't
getting power from the PSU.
So, if the PSU is tested and fine like you siad then you should try making sure the power button is
connected properly to the mainboard and hasn't come loose etc.
And that the PSU is connected properly and snugly.

Post back with more info etc and we'll take it from there