Where is the amp mounted, on the box Im assuming? You left the power, ground and signal wires just laying is so I'm also assuming? For future reference if you are just leaving exposed powered wires laying around even for a short time and you need to use the car, you should heat-shrink them or cover the ends just so something like that cant happen
Definitely before you guy out and buy an amp check everything over. If the amp is 100% on, check to see if the signal cables came loose or possibly something behind the deck came loose, you never know. Also check all your fuses, this is just a standard check, you said its getting power though so shouldn't be an issue.
Really the amp being on just means it got the turn on signal and is grounded, doesn't necessarily mean its getting enough power to push the sub. Check with a MM the voltages and that will give you some more info.
Also just to rule out the sub being a gonner, you can pop a normal 9V battery onto the sub terminals and see if it moves, if its ok it should move. If the sub moves which it should its safe to say its your amp and something went haywire somewhere. most audio shops will take it apart and look at it too see what's wrong for very cheap. Fixing it is another story
hope that helps some.