Originally Posted by caloso

It may not be the bulb, but the switch instead. I had the same Cygolite and the switch is just a plastic slider that makes two metal flanges touch to complete the circuit. One of the flanges can get bent so that no contact gets made. I called Cygolite about it and they suggested taking the thing apart and then bending the contacts gently with a needle-nosed pliers so that they connect more firmly.