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.