More then likely the motor mount screws came loose allowing the motor to drift away and move right or left throwing the gear out of alignment. Alloy motor mounts are a good idea for this model.
To adjust the two gears you need to:
Loosen the motor mount screws slightly so that the motor can slide front to back and slightly right to left.
Place a business card between the two gears.
Point the front of the car straight up in the air.
Push the motor toward the diff gear with light pressure with the card between the gears. The card will be your mesh.
Make sure the gears are flat against each other and not too far to the right or left. (If your gear is worn from riding sideways you will need to replace it. Also check that the diff gear has not been damaged or worn, replace it also if wear can be seen.)
Tighten the motor mount screws. (Check these screws each and every time you run the car until the alloy upgrade can be done. It is also recommended a sealant be installed on the head so you can visually check if they have spun loose.)
Remove the card by turning both tires rotating it out of the gears.
Recheck the mesh of the gears - They should not be so tight as to bind but tight enough so as to not be sloppy. When looking from above the gears should align side to side.
Hope this is what you where asking for.
Good luck!