What car is this? If it is belted, check the belts. Check if they are correctly positioned and not worn. Check for any rocks in between the belts and gears. Check for any rocks in between gears and the chassis.
If it has a axis, check for rocks in (or under) the casing of the transmission. It must be a rock, pal. If not a rock, it's dirt that gummed up and is locking something.
One and only car: HPI RS4 2!!!