Get in your car, fasten seat belt and drive. While driving, undo the seat belt and press & hold the + volume button on the left side of the steering wheel. Hopefully that will fix it for you.
Found the following thread doing a Google search - Voice command button suddenly not working. If you haven't already as is stated in that thread you may want to try a factory reset of the infotainment center (if needed you can find instructions on how to do it here). I had an issue w/my navigation not loading and the reset resolved that problem. One negative thing about doing the reset is you'll loose some of your settings (you'll see in the thread I linked too the ones I could remember that needed to be reset). Good luck and let us know if this corrects the problem.