The Wii sensor is one of the more flimsy parts of the Wii console. You can try to take it apart and search for malfunctions but if you are short on time, there are alternatives. This will tell you how to create your own sensor bar in case it breaks or gets lost. You can use this as a temporary fix until you can get a suitable replacement.
On the other hand, If you are devoted to get your sensor bar repaired. then you should check out this. Here is a great fix for a wii sensor bar that has been damaged buy animals.