The headlamp fails to turn on after rotating the lens piece to the on position. If this is the problem, either the batteries may be drained or defective and will need to be replaced or the 3V incandescent bulb has burned out and will also need to be replaced.
Problem Remains Unresolved
If replacing the batteries and bulb fail to remedy the problem the circuitry of the headlamp may be fried. If this is the case, the headlamp is unsalvageable.