Naturally, make sure batteries are installed correctly. Inspect the batteries to see if the positive or negative terminals are bent in from the fall. Shake the mod with the batteries installed. If you hear any battery rattle then a battery terminal may have gotten bent.
I have had mods that make an insignificant-seeming fall do this because battery terminals would get bent. Look at the battery bottom door to make sure the two terminals are working, and make sure the little contact pin in the door is making good contact. Look up inside the mod and pay special attention to the battery terminals on the other end. to see if they look proper.