Most likely the light on the mobo shows that the 5 volts standby voltage is applied to the mobo. This is ALWAYS the case as long as the power supply is plugged into a live 120 VAC outlet, and the switch on it's rear is on.
If you are in the habit of fully turning the supply off [which removes the 5 volt standby voltage, and disables the power mgt logic], it is possible that the first turn on try after repowering the PS will simply glitch on and then right off again.
If you want to turn the power supply off each time, you can go into the BIOS and enable restart after power loss. If you do that the system will start up without even having to hit the front power switch.
Generally don't assume/guess a specific failure, do some troubleshooting which I assume is why you posted here.
um... im really mad right now cuz on my gaming computer when i plug in the power cord and flip the swich and then hit the power swich nothing hapens when i flip the powe swich in the back the light on the mobo doesnt even come on. It jsut like turned off when i was on it and now it doesnt wan to turn on. Is my mobo shorted out.