Well, it's hard to say. More than likely it's just a bad LED, especially considering that everything is working fine and the LED worked in the past without issue. That would be the 1st place to start. Other things it may possibly be range from a connection that came loose or a solder break to a faulty resistor in the chain, or who knows what else. These aren't as likely though. A good way to find out would be to open it up, get out your digital multimeter, and check to see that you are getting current at the LED leads. From there, you'll know for sure whether or not it's the LED.