Does anybody know what causes a blown subwoofer or amp???

Game head

Hi im having problems with my 505 sub. its been working find for 4 month and then all of a sudden no sound what so ever comes out of the sub not even hum noise. The green light comes on when its connected to my onkyo HT-RC 180 receiver and when i try my signal tests from my onkyo menu i get no sound. I know its not the cable because the green light comes on as soon as i turn on my tv with my sub set to auto. I tried to hook up my sub to a cd player to see if its my receiver but i still get no sound. Im still under warranty with Polk so they are sending me a new one:D, but i would like to know what would cause my sub to just stop working so i can be more educated on the subject. I don't like making the same mistake twice, i need to know if its just bad craftsmanship or something i did. I listen to my sub at 50% volume and set my receiver to out put at +2 db for sub so i don't listen at extreme volumes. Any type of information on this matter would be appreciated. Thanks in advance!