Could be one of several reasons for it not moving, The diff could be blown or not tight enough, If it has a slipper it could be too loose, The clutch could be worn out, The spur gear could be stripped... Best thing to do is to visually inspect everything carefully and go from there. You can also take it to your local hobby store (if there is one) and have them take a look at it, most hobby shops are willing to check things over for free and then give you an estimate for repairs and parts if needed.