Maybe no one can help you, the car isnt the kind of thing a racer would purchase and we are primarily racers here.
Being two wheel drive the car isnt that complex and you probably have to get out a screwdriver and open it up to take a look. Since the car is running in reverse you will have to look at the mechanism that controls forward/reverse movement which is the speed controller. These days most cars have a speed controller but your is more than likely controlled by a servo.
(ie something like this http://www.rcmart.com/catalog/images..._mg995_001.jpg
Note if this is the case there will be two servos in the car one for steering and one for acceleration/reverse.
If all else fails try turning the car on and while it is open and flicking into forward and reverse (if you do this remember to keep the wheels off the ground). And take carefull note of what moves inside the car as you dont want gear flying everywhere when you gas the throtle.
You can also try contacting HPI but i dont know if you will get a response or not, or you may have to take the car to your local hobby store and pay to have someone fix it for you.
Also, dont hold your camera so close to the object it looks like it cant focus on macro (close) shots. I've no idea what im looking at. But the pictures you have taken are pretty much useless anyway as the part has more than likely fallen out from inside the car and you are showing us pictures of the outside.