It sounds very much like a wire has come loose inside the car.
To be sure though take a 9 volt battery and apply it to the braids. If it is still not working take the braid plate off and apply the battery deirctly to the metal pick ups under the braid plate.
If the motor works then likely the braid plate is on upside down and not making contact to the pick ups.
If it still doesn't work you will have to inspect the insides of the car.
The cars with the set come with digital chips in them already but I am not sure if they are the digital chip plugs or if they are soldered directly to the car.
In any case, carefully take the body off by unscrewing it remembering where which screw was removed (they might be different lengths) and make a visual inspection if any cables have come loose.
You may have to jiggle them a bit.
The digital plug is the microchip in the middle of the car. Check it for obvious signs of damage or burning.
If you can, get a 9 volt battery and try to connect it directly the motor. You may need a small piece of wire to do that or someting else that conducts electricity.
If anything seems out of order after all this you should first contact where you got the set from and/or Hornby customer care with the links at the right.