You can try restarting the device to see if it resolves the issue. There could be several reasons why this problem is occurring:
First, check if there are any foreign objects stuck in the left/right wheels or if the socket is loose. If only one wheel is not turning, swap the left/right wheel socket and start the machine to see if the same wheel is still not turning. If it is, then the wheel needs to be replaced. If not, then the motherboard needs to be replaced. If the robot starts to move diagonally, stops and starts, or pauses after hitting a wall, then the motherboard is likely damaged and needs to be replaced.