I bought a sub woofer

Question

I bought a sub woofer from a thrift store used and even though my existng computer speakers have a sub woofer jack, after hooking it up, it doesnt play. I bought the digital chord which fits well in the speakers. Why doesnt it work. I even bought a bigger power supply and the sub woofer still doesnt work, it only has a very low volumed hum. Any suggestions would be appreciated. Thanks. Paul Brady.