|The case button for the power switch may be stuck inwards. |
The switches used for a desktop power or reset switch are extremely reliable, especially the power switch. They're both the same type of switch on ATX cases - momentary contact - on only when pressed in. You can connect the reset switch to the pins for the power switch and visa versa.
However, sometimes the case button that presses on the switch can get stuck such that it's always pressing on the switch enough that it's on all the time. In that case, the mboard will shut off about 4 seconds after the switch has been pressed when the button wasn't stuck initially then becomes stuck, and the mboard will not start up again until the button has been un-stuck
Or in your case.....
- if the button is stuck inwards, the computer will start up after the AC power to it has been removed when the computer has AC power again, then the mboard will shut off in about 4 seconds,
- if the button is still stuck inwards, the mboard will not start up again until you unplug the PS, plug it back in.