It could be a physical button/connector issue. 
Or, hopefully, it could be a software glitch, which can develop over time. I would try manually reapplying the latest firmware, easy to do (see the firmware sticky thread at the top of this sub-forum). You also could try reformatting the player from its System settings, which can clear up mystery issues–note that this will delete user content, and so save anything you want to keep to another device first.
