Sub now very quiet

i hooked up an amp and sub as they should be. it was working perfectly for a week. then i start up the car one day and the sub is very quiet, even with all power to the amp and sub turned up to full volume, hardly any base can be heard. even playing around with the gain and built-in-cross-over on the amp didnt help. whats so wierd is how nothing has changed since it was working to when it wasnt, just suddenly quieter. any help would be appreciated. cheers