First of all, there are many reasons why the Lego gear set does not turn, such as bearing damage, gear winding, gear wear and so on, so I need more information to determine the root cause of the problem. However, it is generally recommended that you check whether the bearings are missing or damaged. If there is nothing wrong with the bearing, you can check whether any gears are too tight or too loose. You can also try to lubricate gears and bearings. If the problem still exists, you can consider replacing the gear or bearing. If you have any other questions, please feel free to ask me.