So I just replaced my rig of five years with a sexy new PC. I had it running (healthy) in the living room temporally until a few days ago when I unplugged it. I finally plugged it back in today and no it won't turn on. The motherboard light is on indicating power but when I hit the switch, nothing happens. Nothing at all - not even any of the fans spinning up.
So, what should I get at first? Is it possible the power switch is simply bad? Is it more likely the PSU? (I don't have a spare PSU to test it.)
Quick test of the power switch and its wires is to disconnect its lead from the mobo's pin connector and short these two pins momentarily with a small screwdriver. If it turns on you'll know the switch etc are suspect.
Sometimes a system just plain gets "confused" and can't respond to a power switch push. Completely disconnect the power cord at the back, leave it off for at least 30 sec, then put it back. Then try the front power switch. That does a complete power-down and cold restart, which sometimes resets things to work again.