I suggested KDR fix his DS (digitizer going bad), so I'll recommend the same for you, it's possible that either the switch on the button is stuck down, or has come loose. It could also be the spring that pushes it back up has gotten free. I'll be more than happy to fix it for you if you want to send it my way. I'd fix it for $10 plus shipping each way, though if you're comfortable with a tri-wing screwdriver (and possibly a soldering iron) I suggest you give it a go yourself. The springs are a pain in the ass to set sometimes, but it's not too difficult.