Unfortunately, there is probably not an easy way to fix it unless the hard black plastic casing can be removed? It may be something like a broken internal wire/solder point from being dropped that connected to the vibrating motor.

Disclaimer: I am not an expert and don't have this specific product. But, from what I've seen with the toys I have is that for the toys to be waterproof and keep the prices lower, there is not usually a way to take apart the toy to access the internal wiring (without destroying the toy and being able to put it back together). One of the last steps in manufacturing is likely encasing the product or filling in the outside of the product with silicone which makes it waterproof by sealing in all the internal electronics/wiring.

For example, we have a wearable vibrator with a thin neck that I bent too much and too often, and one of the vibrating motors no longer turns on. My guess is I broke the wiring in the thin neck, and since the product (Dual Igniter) is encased in silicone to make it body safe and waterproof, there wouldn't be a way to access the wiring without destroying the toy. less