There is anecdotal evidence from numerous other button issue posts (not, and never confirmed by Fitbit) that the buttons can go U/S. Probably poor quality contacts, misaligned waterproof seal or dirt, grime and sweat interrupting the electrical contact surfaces, or the contacts simply not contacting any more - READ: Poor quality.
Best you can do is put a few drops of isopropyl alcohol around the button whilst furiously pressing it to try and work some of this fluid into the contact area to clean the contact. Electrical contact cleaning spray would be ideal if you have access (or can get access) to a can of that.
Your best option unfortunately unless you are within warranty.
