Jl audio 500-5
Assuming that you've confirmed that the speaker is in good working order and that the wiring isn't shorted in any way...
There are separate power supplies for the high and low amplifiers. It's possible that the one for the sub output has failed. If that's the case, the output transistors probably failed and that's what damaged the supply.
Generally, I'm willing to help people repair their amps but this is a large, complex amplifier and it should be repaired by a qualified technician.