I know exactly what you are talking about on this. Does the button actually push in and click, or does it appear to be jammed down in there?
If it pushes in and is not working, it is most likely the button underneath that is soldered on the board has probably worked its way loose, or a circuit has gone out due to wear and tear, either way it would need to be resoldered, the worse case scenario is some kind of moisture or liquid damage. in which case it can still probably be fixed, its just a matter of what all is actually damaged.
If it is physically jammed, the button underneath on the board may still be damaged and may need to be resoldered, but more than likely its just stuck and needs to be opened up and reseated.
I hope this helps.
Please let me know if you open it up and are still unsure of the cause, and possibly attach some pictures of what you see.