A niche is a housing that is permanently mounted in the pool wall, while the light is a “fixture” that is secured into the niche. 

A 12V light receives its power from a transformer, which steps down 120V to 12 V.  120V is your standard house voltage. Some areas of the country do not allow 120V lights. The brightness is equal for either voltage because brightness is measured...

Check the power supply; verify that the breaker in the breaker box and the light switch are in the on position, if all is okay, the bulb maybe blown. 

ALL POWER TO THE LIGHT MUST BE OFF.  This includes the circuit breaker and the light switch. Once the power is verified to be off, unscrew the light fixture from the niche. There should be a single mounting screw in the 12 o’clock position...