Subwoofer Stopped Working

Have two small Velodyne subs in my system connected via speaker binding posts - today one of them not working after four years. It turns on, not muted, crossed over as always, volume set correctly. It's not the amp as I have swapped the other for the failing one in that channel and it works fine. Not the sub's binding posts either as it has two sets and neither works. Have not swapped out the speaker cables but I have literally not touched my cabling in years. What would cause it to have power but yet not produce any sound? All advice much appreciated!