Answer:
No power - breaker, switch off or dead battery
Line voltage below minimum operable voltage (brownout)
Dimmer at minimum so 0% light
Switch/Dimmer faulty
A connection faulty between source of power and bulb
Failure in fixture (Lower Voltage transformer, overheat switch, driver)
Bulb not secured correctly - check base tab for being squished down and cleanliness
Fault in bulb
Safety triggered (overheated component shutdown item)