Matty, it sounds like you have a dying subwoofer amp. If you want to fix it yourself, CAREFULLY remove the woofer to get access to the inside of the cabinet and the built-in amplifier. The amp should have a filter capacitor close to the power transformer. With the AC power disconnected check the filter cap with an inexpensive digital meter that has an ohms (resistance) function on it. Take a 6 inch piece of insulated wire, (a scrap of speaker wire will do), and short across the two capacitor terminals to completely discharge it. Then place the negative lead of your digital meter set to the ohms function on the capacitor's negative terminal and the positive lead of your meter to the center or positive terminal of the capacitor and hold the leads steady as the meter digits increase. This may take a minute or so. You should get a steadily increasing reading on the meter in the Kohm range if the capacitor is ok. If the reading is very low or shows no increase in resistance, the chances are the cap is leaky or shorted. If you feel confident enough to unsolder the cap and replace it get another cap with the same value marked on the old one and the same or slightly greater voltage rating, also marked on the cap. If the cap checks ok, then you probably have failure of some other part in the amp. If you don't have a friend or know someone who can troubleshoot electronic equipment, you had better take it to a repair shop.
One other option you can try BEFORE doing the above - disconnect all audio cables from the sub, then plug the power cord in and turn the sub on with the volume control on the unit turned all the way down. If the green light comes on and stays on then you probably have a bad audio cable or have not connected the sub to your receiver or other equipment properly - recheck all connections and try again . Hope this helps.
radarjohnny