could be a blown sub, could be a bad channel on the amp, could be a bad channel on the deck. Try swapping the rca's on the amp so the left is on the right and the right on the left. If the same speaker continues to not move it could be a blow speaker but it the other speaker does not work then, its something else. If that is the case(the non-working sub changes) then swap the speaker wires on the amp so again the left sub is playing from the right channel and the right from the left. Now if the non-working speaker swaps, you have a bad channel on the amp.
Truth be told the power ratings on subs and the amps to power them is about the most useless spec to look at and I have never once paid attention to the rms/max power ratings on a sub that I was using for my own and also never blown one a sub. Ny advice is to get the best sub you can afford that matches what you want from it meaning do you have the space for a ported/band pass box or are you going small/sealed? After that spend as much as you can afford to send the speaker as much power as you can from the amp, tune it and then just keep the signal clean and out of the clipping/distortion. Don't be worried if you amp puts out more power then the sub is rated for as you can always adjust the gain to keep the power down but then again honestly I will ALWAYS overpower a sub before underpowering it. You can blow a speaker from an amp with not enough power almost as easily as you can with too much. When you over drive the sub/speaker and it starts to distort no matter with too much power or too little it will "clip" the the top of the nice smooth signal wave with a flat line, and does this sooner with an amp that does not have enough power. When that happens the speaker will drive either forward or back to its furthest distance and then hold the speaker there until the clipping stops. When this happens the speaker will start to build up heat, and also not be able to disapate the heat as fast since it is not moving which is how it is designed to cool itself. When it does that enough times it will get hot enough to start melting something. Like I said now with a lower powered amp you will reach that clipping point much sooner then with a higher power amp so you would need to drive it for a little longer to build the necessary heat but it is possible to blow a sub with too little power just about as easy as with an amp with too much power. Again thow as much power at it as you can affford, listen to the music, when it starts poppin/crackling at higher volumes where it isn't at low volumes its time to turn it down as you are at that point possibly doing damage to your gear.
if that subwoofer was blown it would still move but would sound/smell like shit
