Before you get started, make sure all equipment is turned off, including the subwoofer. You never want to connect or disconnect any cables while anything is on, lest something incurs accidental damage.
If Nothing Works, Consider Replacing Your Subwoofer
If the subwoofer doesn’t power on at all, or it powers on but won’t play a thing, then there’s a good chance that it is defective and needs to be replaced. The inputs on the back of the subwoofer generally plug into the subwoofer output on the back of receivers or amplifiers. If the subwoofer connects to speaker outputs on the receiver or amplifier, inspect the entire length of wire connections for defects. If any bit of wire appears to be worn, torn, or damaged, replace those wires before attempting to use the equipment again. Perform a quick test on the wires to check that they work. As with the speaker wires, inspect the subwoofer’s power cable for any damage or defects. Some subwoofers are equipped with a fuse, which may or may not require removal of a back plate. If the fuse is a feature, and if you’re comfortable tinkering with electronics, check to see if it needs replacing. Otherwise, consult the manufacturer or local repair shop first. If the input device offers speaker size settings, choose the smallest option first; sometimes setting the speaker size to something larger makes it so that the subwoofer doesn’t receive a signal. Some receivers will, in fact, allow subwoofers to operate with a large speaker setting, so consult your product manual for additional details. If possible, connect a separate subwoofer to the sending device to test that the hardware malfunction isn’t related to the sender. If the second subwoofer works, then it’s very likely the original is indeed bad. But before you start shopping, determine if you need a powered or passive subwoofer.