Push down on the cone (gently, only need about an inch of movement), is it a smooth feeling? If not, the sub is rooted... There is another way the sub can die, the two wires going to the voice coil can snap (I've done that a few times), IF thats the case, get some wire and reconnect them...
Can you test it with another sub?
At this point it could be the amp, sub or the signal from the RCAs... If a different sub works, then it is deffinately your sub...