How to test if electric is dead? Accelerated forward while rolling back.

[Enfollc]

My son had the cart out today. He was parked on a little hill and put it in reverse, then as the cart was rolling back, he put it in forward and pressed on the accelerator. The kids said it made a noise then it stopped. They could smell something "hot or burning" from the cart. I went outside and the cart doesn't go forward or reverse. I can hear the selinoid clicking but it doesn't move. Where should I begin testing? What possible damage could they have done? Hoping they didn't damage the motor. I already dropped $600 in batteries this year. Thanks!