If it's the stock motor, then you've got a couple options.
One, replace it with another stock motor (about $5)
Two, upgrade it to the Reedy mod motor (about $35 or so)
Or three, brushless time!! (about $160, if I recall right)
Before you go any of the three above routes, check your motor by the following; try rolling the car on the ground. You should feel a faint yet smooth "cogging" effect. It should be consistent. This is the motor rotating in the magnetic poles.
If the car will not roll, chances are you knocked a magnet loose in the motor. To verify this, get a jewelers phillips screwdriver (that's what you need to work on a car this small, lol), and take out the motor. Once the motor is out of the car (it's four screws and it comes right out), repeat the rolling test again. The car should roll smoothly and freely with zero resistance.
Remember, if you do replace the motor, to set the gear mesh accordlingly. Too tight, and it will bog the car down and cause it to crawl.