In the case that your controller has a button that's stuck, then that's most likely caused due to build-up grime and dirt around the edges on the button. Our advice then is to clean the button for a moment carefully with a little cotton swap with a bit of rubbing alcohol. Rubbing alcohol evaporates really fast and will not damage the internal electronics of the controller.
Dip your swap in rubbing alcohol and clean the stuck button around the edges. Press the button a few times and clean the edges once more. Repeat this for about 5 times and then the button should be functioning again.