You can send it to nintendo, or as mysticalZoey said blow in it (results not permanet), you have dirt in it or some forgin stuff, and you couldve used it so much there shot. but your best bet is blowin.
Yes. More than 90% of the time, it's just some dirt getting under the button pads. You can alleviate this by forming a seal around the button with your lips and blowing a few times.
Very, very rarely is this a result of the solder coming loose or a frayed wire inside. In which case you are sure that the blowing solution did absolutely nothing, you'll want to ship it in for repairs.
I've been doing some research on this problem, and so far I've found that blowing into the button is a temporary solution.
When you blow air into the space between the button pad and the case, what you're actually doing is moving the metal clip that holds the button's underside to the frame. Unfortunately, the moisture that is incurred may damage the sensors and rust the clip. Seeing as the actually problem is the clip becoming old and not moving properly, you might consider compressed air to get it back to functionality.
So while blowing does work for a time, it could eventually become necessary to have the parts replaced.