Oli_hd is right. The 7407 (U352, an inverter) switches the ground side of the LED through a 56 ohm resister (R300). The cathode is tied directly to a branch of the +5V rail.
Can you get your hands on a meter and check the power supply for +5V? Check the little board with LEDs for +5V? Check where the wires from that board ties to the MB? Check this branch all the way to where it is supplied with +5V by the Power supply connector? If you burned out a skinny +5V trace that supplies the LED and several chips, you will have to solder in a new wire to re-supply this branch with +5V.
It's sorta like heart bypass surgery. Grin.
You didn't put the LED in backwards?
