Remove the Subwoofer RCA connector from the receiver side and touch the tip of the RCA connector with your finger, the sub is supposed to make a humming noise, just be sure that the volume control in the subwoofer is up.
If it makes the humming noise, check the subwoofer settings in the receiver, you will need the receiver user manual to do that, you can't figure it out, if you do not have one purchase one on line.
If the sub does not make any noise when you touch it, replace the cable and try again and if still does not work, replace the sub, most of the times is not worth it to send them for repairs.
You may understand that I do not have the unit in front of me and I can't remember every detail of all the units out there, if your unit does not have volume control, then they have pre-set the volume and anyway is supposed to hum, if the cable is broken you would need to find another anyway or if you do not want to get into that trouble replace the complete unit.
Subwoofers has digital amplifiers inside and there are no parts that you can replace to make it work again, even the repair centers replace the whole module when they fail.