|On the motherboard front panel header there will be a pair of wires that connect to the PWR LED. I suspect one of those wires has either come off it's pin or has a loose connection. |
In some systems the front panel connector is a single block with all the wires running into it rather than separate wires connected to each header pin. Sometimes the connections inside the block can be a bit loose. In any event you will only find out by opening the case and checking the connections.
Goin' Fishin' (Some day)