I see. Anyways, there are multiple possibilities here, such as a faulty power button, power supply, or motherboard.
There's a very simple way of bypassing the case's power button and turning it on directly - simply disconnect the power button's cable from the motherboard's front panel headers, and then short both of the pins with a flat bladed screwdriver (touch em both at once for a moment with the tip and release) and see what happens. Do note that in most cases, the power button's cable would be using green/white or green/black wires. You could also lookup your motherboard's PDF manual to give you more specifics if needed. If it turns on, you could just put the reset button's cable in there instead (that way the reset button would be used for power), you'd no longer have a reset button though (which is not a biggie, as the pc would turn off if you hold the power button for a few seconds) but if it doesn't turn on, then that would leave the power supply or motherboard. In order to test the power supply, disconnect the power cord, remove the main (20/24 PIN) connector from the motherboard, then use either a metal paperclip (or a wire stripped on both ends) and put one end in one of the blacks, and another in the green, in similar to the following picture:
http://www.techwarelabs.com/guides/misc_mod/psumod/images/image012.jpg , better put it down on a piece of cardboard when done (for extra safety), then plug the power cord back and see how it goes (the PSU should automatically turn on if it's in good shape).
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